Resilient xaml (#4096)

Resilient xaml (#4096)

* [Xaml] un-obsolete XamlLoader.Create

now that we plan to load Xaml files without compiling the ctor, keeping
XamlLoader.Create makes sense.

* [Xaml] add FallbackTypeResolver

* Additional tests

* More tests

* Further tests

* [Xaml] provide an mock App

* [C] allow re-registering implicit styles

* Some tweaks and reinstated test with missing resource key

* Add StyleAppliedToDerivedTypesAppliesToDerivedUnknownType test

* Renamed some stuff for consistency. Added some CSS tests.

* Remove use of ValueTuple

* Temporarily disable failing tests

* redirect valueprovider exceptions to the handler, allow the handler to intercept excpetion with Create()

* fix the CSS situation

* new callback, works for CSS