Use soft DataContext synchronization for Reactive UI controls

[jp2masa]

What does the pull request do?

Improves the behavior of ReactiveUI controls by synchronizing DataContext and ViewModel only when it makes sense.

What is the current behavior?

Synchronization between DataContext and ViewModel is a bit aggressive. For example, setting ViewModel = null unsets DataContext, which becomes inherited and breaks compiled bindings. Also, at least on Windows Forms, WPF, and MAUI, these controls don't synchronize the properties at all.

What is the updated/expected behavior with this PR?

DataContext and ViewModel can be set independently: when one is unset, they are synchronized.

EDIT: DataContext and ViewModel can be set independently: when one is updated and its old value is equal to the other, the other is updated as well.

How was the solution implemented (if it's not obvious)?

If !IsSet(one) then SetCurrentValue(one, other).

EDIT: If Object.ReferenceEquals(oldOne, other) then SetCurrentValue(other, one).

Checklist

• Added unit tests (if possible)?
• Added XML documentation to any related classes?
• Consider submitting a PR to https://github.com/AvaloniaUI/avalonia-docs with user documentation

Breaking changes

May break expected behavior.

[maxkatz6]

Looks good to me but won't backport.

[maxkatz6]

@jp2masa it looks like ReactiveUI related tests are failing now.

[avaloniaui-bot]

You can test this PR using the following package version. 11.1.999-cibuild0044414-beta. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]