MultiTarget updates: Organize GlobalUsings, add HasWinUI2 msbuild property

[Arlodotexe]

This PR:

• Organizes our GlobalUsings
  • Cleans up random imports or exclusions
  • Adds a EnableGlobalUsings property (enabled by default)
    • Respects the HasWinUI property when importing GlobalUsings.WinUI.cs.
    • Removes imports in various files that no longer need it
• Adds a HasWinUI2 property to complement our existing HasWinUI property.
  • Can be used to enable or disable references to Microsoft.UI.Xaml when running under UWP without turning off references to WinUI (wasdk, uno) on other deployable targets.