[Enhancement] Add CollectionView progress behind feature flag (#4265)

[Enhancement] Add CollectionView progress behind feature flag (#4265)

* The barest beginnings of starting a spike for LV2

* Very basic RecyclerView

* Very, very basic UICollectionView

* Start setting up galleries

* De-duplicate some gallery code

* Android horizontal layout

* Very basic horizontal/vertical text lists for iOS

* Vertical list views working in iOS, stashing while trying to figure out what the deal is
with horizontal views and frames with heights of 1

* Working on rotation issues in iOS

* Fix name of renderer in UWP; make horizontal scrolling work for horizontal
lists on UWP

* Set up gallery to generate different sizes of lists for testing

* Respond to items source updates on UWP

* Add gallery for GridLayout; Add GridLayout;
begin GridLayout implementation UWP

* Basic UWP gridness

* Add demo page for grid layouts with text;
Handle grid layout spans on UWP; handle grid layout span changes on UWP;

* Implement span updates on Android grid layouts;

* Ironed out most of the iOS text cell layout issues

* iOS vertical/horizontal text lists now working reliably;

* In progress on UWP DataTemplate stuff

* Dumping the data template into the itemsource

* Vertical list basically working with DataTemplate; horizontal list having image size issues

* Clean up names and move classes to their own files

* Adding some notes for FormsContentControl

* Rolling back platform test on UWP

* Fix Tizen build

* Fix typo and roll back old test code

* Remove old testing comments

* Make basic data templates work on Android

* Update measure to work horizontally

* Remove old testing code

* Terser rectangles

* Use nameof()

* Rename FormsContentControl to ItemContentControl

* Use CreateRenderer instead of GetOrCreateRenderer

* Clean up TemplateCodeCollectionViewGallery

* Add grid galleries for templates

* Nicer example templates

* Create gallery to test removing items from collections

* Smooth single item removal on Android

* Smooth item insertion on Android

* Observable collection item adds working on UWP

* UWP Remove

* Split files

* Add gallery for snap points; start implementation of snap points on Android;

* Move snap handling to separate class; implement Start snapping (horizontally)

* Debugging vertical overlap issues on Android

* Temporarily add IsClippedToBounds to generators to force clipping on Android

* Start and End SnapHelpers are working horizontally and vertically

* Consolidate some snap code on Android

* Mandatory Single Snapping working for Center/Start horizontally

* Throwing together scrollto gallery

* Super basic "ScrollTo" implementation for Android, plus test page

* Expand ScrollTo test harness;

* Move ScrollToRequestEventArgs to its own file

* Mostly working Carousel renderer; needs some cleanup and its own gallery pages

* Carousel gallery

* Naming cleanup

* Make parameter name consistent

* Finally some basic data template functionality for lists

* Checkpoint; sort of working

* Working out estimate size hacks

* Working out cell optimizations

* Checkpoint before some serious refactoring

* Collapsing closer to a solution; text lists and grids working;
template lists and grids working; uniform size hint mostly implemented;

* Remove awkward Update method

* No longer hard-coding list item heights estimates

* Consolidate more code

* Get rid of event on ItemsViewLayout

* Remove public NeedsEstimate stuff

* Get rid of IConstrainedCell

* Watch for Span changes on iOS

* Indexed ScrollTo on iOS

* Implement removal/insertion of items on iOS

* Handle Move, Reset, Replace on Android

* Add screens to test ScrollTo Item; implement ScrollTo Item for iOS

* ScrollTo Item working on Android, but only works for MakeVisible

* Implement Android ScrollTo Animated ScrollToPosition

* Ugly but functional non-animated scroll with ScrollToPosition

* Move Scroll logic into helper class for clarity and maintenance

* Fix Start snapping with grids (hard-coded span of 1)

* Fix next/previous row/col snapping bug for EndSnapHelper

* Remove some debug statements

* Clean up example code

* Fix missing delegate setting on CollectionView

* Add filtering example

* Watching the adapter and handling empty view from the renderer

* Add debugging for all the FindViewHolder methods

* Suuuuper basic but functional EmptyView

* Allow EmptyView to be turned on and off

* Building tests for all the EmptyView scenarios

* Set Platform on EmptyView (for now)

* Templated EmptyView on Android

* Clean up merge errors

* Require flag to use CollectionView on Android

* Require flag to use CollectionView on UWP

* Restore Label functionality commented out during testing

* Adding a note for myself

* Add flags for iOS; flags for Android CarouselView;

* Remove Calabash out-commenting

* Undo changes to DotSettings; remove outdated TODOs

* Fix formatting

* Remove debugging code; remove outdated todos; remove unnecessary comments

* Undo randon VS changes

* Include CollectionView in experimental renderer testing

* Use correct property in nameof() call

* Rename ScrollToRequestEventArgs Animate -> IsAnimated

* Rename Constrain -> ConstrainTo for consistency

* Add Core flags feature; add flag verification for CollectionView features

* Make sure no one is subclassing native CollectionView stuff without flag

* Set CollectionView flags for unit tests

* Make flags depend on Application.Current

* Fix iOS 9.3 exception when laying out CollectionView

* Use Device.Flags

* Move CV flag check to CollectionView; use Device.Flags on native platforms;

* Move classes into their own files

* Make CollectionView flag check method non-browseable

* Remove old TODO

* Remove Element.Platform sets

* stubbed toolbox entries and snippets for Carousel and Collection

* updated CV snippet to include layout

* Remove Linq using statement

* Remove unused using statement

* Turn off CollectionView in Control Gallery

* Move PropertyChangedEventArgsExtensions to internal on each platform

* Clean up debugging code

* Fix broken test setup

* Remove ToString overrides

* Fix broken test screen
Fix broken test screen

* Fix rebase errors

* Update submodule

* Fix rebase error