Better main thread support

[madsmtm]

Our main thread support in MainThreadMarker and MainThreadBound is still a little akward to use, and is missing some functionality that would be nice.

TODO:

• Automatically add marker to required methods: Sendability attributes #359
• Allow calling non-isolated methods off the main thread (see this)
• Add main thread methods to protocols in header-translator: Fix main thread requirements on protocols #509
• ...