RelativeSource Binding (#4375)

RelativeSource Binding (#4375)

RelativeSource Binding

- fixes #3847

Implement RelativeSource binding modes for Self, TemplatedParent, and FindAncestor. Add unit tests. Pare down TemplateBinding class to use RelativeSource binding mechanism.

* Include RelativeSource member in Binding.Clone

* Fix issue of unit tests being tricked by async in Binding.Apply; permit "Source={RelativeSource...}" in addition to "RelativeSource={RelativeSource..."; allow FindAncestor relative binding source mode to be based on BindingContext type or control type

* Fix unit test wasn't actually testing "Source={RelativeSource...}" syntax.

* Address @StephaneDelcroix's comments. Make FindAncestor relative source binding work even when the ancestor changes.

* Much better mechanism for detecting when a parent changes for relativesource

* Cleanup

* more cleanup

* more cleanup

* Remove async/await for identifying templated parent and ancestor type binding source

* Remove unused code; fix linting

* Update Xamarin.Forms.Core/Binding.cs

Co-Authored-By: Stephane Delcroix <stephane@delcroix.org>
* Update Xamarin.Forms.Core/Binding.cs

Co-Authored-By: Stephane Delcroix <stephane@delcroix.org>
* Update Xamarin.Forms.Core/Binding.cs

Co-Authored-By: Stephane Delcroix <stephane@delcroix.org>
* Update Xamarin.Forms.Core/BindingExpression.cs

Co-Authored-By: Stephane Delcroix <stephane@delcroix.org>
* Update Xamarin.Forms.Core/BindingExpression.cs

Co-Authored-By: Stephane Delcroix <stephane@delcroix.org>
* Update Xamarin.Forms.Core/Binding.cs

Co-Authored-By: Stephane Delcroix <stephane@delcroix.org>
* Ensure TemplateBinding works correctly when reparenting. Cleanup.

* Finish addressing comments; linting cleanup

* Remove private modifier where unneeded