Better menus, worker queue, branch deletion, multi selection

[smlpt]

This PR adds the following new features:

• branch deletion by holding the delete button for 0.5s
• multi selection: simply click another sphere to add it to the selection. Dragging with the selection button allows you to quickly "paint" a selection. Clicking away will remove the selection
• Graph changes are now handled asynchronously through a worker queue that locks mastodon's graph while editing

Fixed:

• timepoint wrapping between TP 0 and maximum TP
• always include the whole time range for NN linking
• unified the start/stop VR buttons into a single button to prevent state issues
• Track merging is now handled more robustly
• Fixed polarity of tracks that are sent to Mastodon
• Fixed spot radius representation in sciview by calculating the geometric mean for each spot. This maintains the same volume for spheres in sciview even if the mastodon spot has an ellipse shape (and we currently don't support ellipses due to a weird issue in representing the rotation)

[smlpt]

@skalarproduktraum due to lack of admin rights in the repo: please review :)

[smlpt]

The only aspect I'd like you check for the review would be whether I implemented the worker queue correctly.

[smlpt]

@skalarproduktraum I added a few minor bug fixes to this PR, but now its really ready for review.