Context menu 4: add "Clone space view" action

[abey79]

What

This PR adds "Clone Space View" to the context menu (when clicking a single space view). Also, now the the cloned space view appears just after the original space view, in the same container (previously it would typically appear at the end of the root container regardless of the position of the original space view).

This PR also unearthed a bug that I'm surprised didn't surface earlier. The effect of mark_user_interaction wasn't visible until the next frame, as the value was only saved to blueprint, and not cached in the ViewportBlurprint. This could lead to an issue when TreeAction are processed end-of-frame and exposed to the "old" mark_user_interaction flag values (namely: unwanted reset of the tree). Those flags are now Mutexes (to gain interior mutability) AtomicBools and updated in place (in addition to being saved to blueprint like before). cc @jleibs

part of:

• Add contextual menu to blueprint tree for common actions #4823

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[abey79]

Mmm I get some weird behaviour with opf (after bp reset): the cloned space view ends up in a newly created grid container 🤔

Edit: ✅

[Wumpf]

The mutex handling indeed doesn't make sense to me. Everything else checks out (:

[Wumpf]

much better :)