[Experiment] Implement EventSetter

[maxkatz6]

What does the pull request do?

I was wondering how hard it would be to implement something like this.
After a bit I found two main problems:

1. Our ISetterInstance infra expects anything that has a value. EventSetter doesn't have that. Had to implement IValueEntry in a dumb way to make it work. Essentially, we need some ISetterInstance interface that is able to call Unsubscribe.
2. RoutedEvent lookup is problematic. Correctly it should use RoutedEventRegistry and lookup by actual event name. On practice we can't do it compiler time and validate it. This PR implements a hack that look-ups by event definition name that must exist in target or base types. Alternatively, user can use x:Static which will work even for attached events.

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

Fixed issues

Fixes #12627

[avaloniaui-bot]

You can test this PR using the following package version. 11.2.999-cibuild0047292-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[kekekeks]

I'd prefer something typed here rather than a plain Delegate. It kinda affects perf and had problems with NAOT previously (probably reflectionless one, don't remember)

[maxkatz6]

Shouldn't matter here.
Compiler still generates a typed EventHandler<RoutedEventArgs> instance. Which should give enough information to NAOT compiler.

It's then downcasted to Delegate, but our routed events infra uses Delegate internally anyway.

[kekekeks]

There were problems with invoking the delegate, IIRC

[maxkatz6]

Well, I sure can make EventSetter a generic type. And wire it up with some compiler magic.

[maxkatz6]

But I probably won't spend much time on it, unless @grokys approves these ISetterInstance hacks.

[jmacato]

@cla-avalonia recheck

[cla-avalonia]

• All contributors have signed the CLA.

[maxkatz6]

@cla-avalonia agree