3D->2D & 2D->3D selection visualizations stick now around on selection

[Wumpf]

What

• Fixes: 3D->2D & 2D->3D visualizations (depth display & ray) should stick around on selection #3047

Makes the 2D->3D and 3D->2D sticky upon selection!
Going the extra mile to make selection history be aware of this added a bit more stuff, but the code is better off overall I think :)

sticky.projections.mp4

Update! Now also works fine with multielection!
We now associate each item selection with an optional space context. Right now you can't space context without items, but this is not a structural limitation.
Toggle (ctrl+cmd select action) behavior has a few new special rules now: If you change the space view context on an already selected object its context is updated. If you add a new object, we add it to the list with the context.
Overall this paves also the way to displaying the context together with the selected item in a more sane way. Should be easier to reason with in general I believe.

Screen.Recording.2023-12-20.at.13.10.22.mov

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[teh-cmc]

Nice! Multi-selections doesn't seem to work though (whether it's multiple rays in the 3D view, or multiple depth markers in the 2D view)? Is that expected?

[Wumpf]

Yeah that's expected so far. There's some curious asymmetries in the code about how this works out for hover & select. But I reckon it wouldn't be hard to set right 🤔:

• a selection should be allowed to have several "space view contexts" and a hover only have one
• adding entities to a selection shouldn't remove existing context objects and may add additional ones
• history always stores a vec of contexts

[Wumpf]

Let's do it. It would be pretty neat for multi point in the 3d viewport