Add Support for Prism.Avalonia v12.1

[DamianSuess]

Description of Change

Adds support for Avalonia v12.1 to Prism.Avalonia, and Dialog box example.

See also, #3391

Bugs Fixed

• Fixed Dialog Box not requiring "Title" property, as Avalonia enforces compiled bindings.
• Avalonia.Diagnostics has been deprecated, references the new AvaloniaUI.DiagnosticsSupport

API Changes

N/A

Behavioral Changes

As of Avalonia v12.x, the Avalonia.Diagnostics project has moved to AvaloniaUI.DiagnosticsSupport

PR Checklist

• Has tests (if omitted, state reason in description)
• Rebased on top of master at time of PR
• Changes adhere to coding standard

[DamianSuess]

looking into the build issue...

I am noticing when locally running all the tests that the following two tests will fail. However, when executed by themselves, it works just fine.

• RegionGetsCreatedWhenAccessingRegions
• RegionWillNotGetCreatedTwiceWhenThereAreMoreRegions

[DamianSuess]

(Update on the Fix):

Updated DelayedRegionCreationBehavior to use targetElement.CheckAccess() for Avalonia too (same as other targets), instead of Dispatcher.UIThread.CheckAccess(). This removes the dispatcher-global gate that prevented region creation in the failing test path.

[DamianSuess]

@brianlagunas and @dansiegel, I got to sync up with you guys regarding NuGet.org so we can migrate ownership of the packages. We can chat about this via Teams or email if you'd prefer.

[DamianSuess]

Thank you sir!