Remove disposable allocations from routed event AddHandler in the common case.

[MarchingCube]

What does the pull request do?

AddHandler was always returning IDisposable allowing for easy unsubscribe for the user. This PR changes this behavior to not do this by default and adds a separate function with this behavior.

What is the current behavior?

Most of the AddHandler callers don't even use returned value (all event implementations in AvaloniaObject just discard returned IDisposable). There are only a few call sites that use this feature.

Method	Mean	Error	StdDev	Gen 0	Gen 1	Gen 2	Allocated
CreateCalendar	19.29 ms	0.1664 ms	0.1557 ms	625.0000	281.2500	31.2500	3.61 MB

What is the updated/expected behavior with this PR?

Adds a separate AddDisposableHandler that can returns IDisposable that can be used to unsubscribe from the handler.

Perf gains in calendar case are tiny but we can clearly see that we collect less often now as AddHandler is not forced to allocate all the time. But we shouldn't prevent users from not being able have non-allocating AddHandler to support 1% niche use case.

Method	Mean	Error	StdDev	Gen 0	Gen 1	Gen 2	Allocated
CreateCalendar	19.18 ms	0.2813 ms	0.2631 ms	593.7500	250.0000	-	3.6 MB

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

Checklist

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

Breaking changes

Users that are utilizing AddHandler return value should use AddDisposableHandler instead.

[grokys]

👍 WPF's AddHandler doesn't return an IDisposable, so removing it and adding an extension method that does makes sense from an API standpoint.