Proofreading without skeletons

[daniel-wer]

Allow to proofread by clicking on segments instead of agglomerate skeleton nodes.
The position of the active segment is now updated when clicking onto a segment and also highlighted in proofreading mode using a small white cross hair. Afterwards, users can right-click somewhere else and select the entry to split or merge (a) segment(s).
Additionally, the active segment is highlighted a bit more than the currently hovered segment to make it more prominent and more easily recognizable.

URL of deployed dev instance (used for testing):

• https://proofreadingwithoutskeletons.webknossos.xyz

Steps to test:

• Create an annotation, activate an agglomerate view mapping and select the proofreading tool
  • Click on some segments - the click position should be highlighted with a zoom-independent cross hair + the active segment should be highlighted with higher opacity
  • Right-click somewhere else and select "Merge with active Segment" or "Split from active Segment"
• Open datasets with different scales - the cross hair should always have the same size.
• Use "Shift + Leftclick" and "Ctrl + Leftclick" to merge/split segments without the context menu. The description in the bottom bar should adapt automatically.
• Loading agglomerate skeletons and deleting/adding edges should still work for proofreading, although it's less discoverable (by design, might be removed in the near future)

Issues:

• fixes [Proofreading] Make agglomerate skeletons unnecessary #6507

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(Please delete unneeded items, merge only when none are left open)

• Updated (unreleased) changelog
• Ready for review

[philippotto]

Great work 👍 Feels already pretty good! See my comments about the overloading of the left-click. During testing, I clicked on a node which was weird, because the proofreading position was set to the click position, but afterwards the z-coordinate was changed to the node (due to clipping distance > 1). Therefore, I couldn't see the crosshair, anymore.

[philippotto]

Excellent 💯 Also thank you for fixing the quick select geometry at 0,0,0 🙏

[daniel-wer]

I'll merge this on Monday in case anything goes wrong :)