Iterate on (skeleton) toolbar

[philippotto]

Also see https://www.notion.so/scalableminds/Design-Doc-UI-and-Shortcuts-76a59ceaa7fb49d8810df24fd56f1282.

There is currently no dedicated skeleton-tool (instead it's in the move tool). This could be separated so that the resulting tools would look like this:

• Move (use Hand-icon)
• Skeleton
  • Click to create
  • Drag to move node
  • Drag to move canvas
  • 'New tree' button
  • 'Soma clicking' toggle
• Brush
  • Drag to brush
  • Sub Tool: Erase
  • Sub Tool: Pick color
  • 'New cell' button
  • Overdraw toggle
• Trace
  • Same as brush
• Fill Tool

To do:

• refactor toolbar so that it's also visible in skeleton-only mode
• implement the above tools
• pay attention that the legacy keybindings still work (also see Re-think keyboard and mouse shortcuts #4972)

[philippotto]

This has been implemented by now.