Invalidate shapes changing any property inside the brushes (Fill, Stroke)

[jsuarezruiz]

Description of Change

Invalidate shapes changing any property inside the brushes (Fill, Stroke).

Issues Fixed

Fixes #13891

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[jsuarezruiz]

@jonathanpeppers
I've added more changes because the proxy we used to detect brush changes in Shapes required to detect changes from other events (changes in brush gradientstops etc.). I have also added a new example to validate this.

Could you review it again?. Thanks!

[jonathanpeppers]

I think I found an issue in this pattern, if you change this to:

Suggested change

	stroke.Color = Colors.Green;
	Assert.True(fired, "PropertyChanged did not fire!");
	await Task.Yield();
	GC.Collect();
	GC.WaitForPendingFinalizers();
	stroke.Color = Colors.Green;
	Assert.True(fired, "PropertyChanged did not fire!");

Does it fail? I found saving some delegates in a member variable fixes it, but looking how we can do it in a nicer way.

[jonathanpeppers]

Follow what I did here to get the above test to pass: #13997

[jsuarezruiz]

Right, the test was not passing in that case.
Applied changes to save the EventHandlers in a member field.

Now are passing (at least locally).