[Enhancement] Sheet - Notify when sheet has ended its animations for open / close

[haavamoa]

Summary

• Add commands that should trigger when the sheet has finished animating and are opened / closed.

API Changes

    <dxui:SheetBehaviour IsOpen="True" OpenedCommand="{Binding MyCommand}" ClosedCommand={Binding ClosedCommand}" />

Intended Use Case

This is useful if you want to wait for the view to be completely visible before doing async calls or if you are using a component that is forcing the view to freeze.