DataGrid fixes for WinUI 3 usage

[RBrid]

A few changes for the WinUI branch so that the DataGrid control can be used in a WinUI 3 app:

• addressed some compilation warnings (casting event args to Control/ContentControl/FrameworkElement as recommended by compiler)
• removed all usages of Window.Current which is null in WinUI3:

• replaced usage of Window.Current.CoreWindow.PointerCursor with CoreWindow.GetForCurrentThread().PointerCursor
• replaced usage of Window.Current.CoreWindow.GetKeyState with CoreWindow.GetForCurrentThread().GetKeyState

PR Type

• Bugfix

What is the current behavior?

Application crashes because Window.Current == null causes null ref exceptions as soon as user tries to interact with the DataGrid.

What is the new behavior?

No more exception, as in regular UWP.

PR Checklist

• Tested code with current supported SDKs
• Pull Request has been submitted to the documentation repository instructions. Link:
• Sample in sample app has been added / updated (for bug fixes / features)
  • Icon has been created (if new sample) following the Thumbnail Style Guide and templates
• Tests for the changes have been added (for bug fixes / features) (if applicable)
• Header has been added to all new source files (run build/UpdateHeaders.bat)
• Contains NO breaking changes

[ghost]

Thanks RBrid for opening a Pull Request! The reviewers will test the PR and highlight if there is any conflict or changes required. If the PR is approved we will proceed to merge the pull request 🙌

[azchohfi]

Nice!