[Core] Make BindableLayout handle collection changed on the main thread

[GalaxiaGuy]

Description of Change

Changes the BindableLayout.ItemsSourceCollectionChanged method to handle changes on the main thread.

Issues Resolved

• fixes [Bug] BindableLayout does not marshal ObservableCollection.CollectionChanged to MainThread #14039

API Changes

None

Platforms Affected

• Core/XAML (all platforms)

Behavioral/Visual Changes

None

Before/After Screenshots

Not applicable

Testing Procedure

There are unit tests that just confirm that BeginInvokeOnMainThread is called.

PR Checklist

• Targets the correct branch
• Tests are passing (or failures are unrelated)

[jfversluis]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[jsuarezruiz]

@hartez What do you think, would you have any objection to this change?

[thisisthekap]

@jfversluis Any chance that this one is making it into a Xamarin.Forms service release?

[jfversluis]

Now that we're so close to the sunsetting of Xamarin.Forms unfortunately we won't be able to take this in anymore, we're really sorry about that. Nevertheless, thank you so much for your time and effort that you have put into this PR.

Please have a look at the evolution of Xamarin.Forms, .NET MAUI. A lot of development has been going on there. Hopefully this issue was already fixed in that codebase. If not, feel free to port this over to there.

Again, thank you so much for being a contributor and Xamarin.Forms user!