BUG: Multi selection over 2 asset sources creates issues

[Sebobo]

When having a local and a remote asset source it is currently possible to multi select assets from both sources.

This leads to problems, f.e. when the remote asset source is readonly and the Ui allows changing the copyright or deleting them, which would of course lead to errors.

So either the selection needs to be contextual bound to the asset source, or we reset the selection when switching sources (which could have been an accident, so we would need to ask the user, if they want to switch and give up their selection).

[Sebobo]

Decided to make all selectors and states asset source aware. Quite a bit of refactoring but not so hard to test and verify. This means that switching between asset sources uses separate selection context except for the clipboard which supports selections across asset sources.