[WIP] Multiple selection

[Hummer12007]

This PR enables selecting multiple regions at once.

Demo: https://l.sr.ht/NU-U.mp4

Implemented:

• [x] Ctrl+Click extends the selection with a new region
• [x] Delete deletes all selected regions

TODO:

• [ ] Fix deselection
• [ ] Make sure move/resize/etc. actions modify all selected boxes
• [ ] Add a tool (or hijack Ctrl/Shift + drag) to select regions with dragging
• [ ] Add a knob to disable this behavior, if necessary
• [ ] Ensure this doesn't break UniversalDataTool
• [ ] Agree with Severin on behavior

[seveibar]

Very cool @Hummer12007! Thanks for publishing your progress!!!

Thanks for the detailed description, looks great so far!