iOS: Frame with HasShadow set to true and BackgroundColor alpha < 1 casts shadow...

iOS: Frame with HasShadow set to true and BackgroundColor alpha < 1 casts shadow on all child views (#7518)

* Updated renderer with additional ShadowView

* Added buttons to test some behavior

* ShadowLayer background color tweaks

* Resolved flawed merge, restored _Template page

* Apply suggestions from code review

Co-Authored-By: Gerald Versluis <github@geraldversluis.nl>
* Added more descriptive issue Title

* Updated Issue5108.xml's properties

* Update Xamarin.Forms.Controls.Issues.Shared.projitems

Fixed missing closing tags

* Updated projitems

* Removed eventhandlers from XAML

* Update Issue5108.xaml.cs

Added eventhandlers in code + surrounded code-behind with APP preprocessor directive to avoid issues when building for UITEST