[Cooja] (Multi-)Mote visualization updates

This introduces several fixes and extensions to Coojas `Visualizer` plugin.

- Fix: Selected motes were highlighted only if UDGMVisualizer was active.
   Now a Mote is always highlighted with a blue circle and a semi-transparent gray overlay which still allows other skins to modify the mote fill color
- Enable multi-mote deletion
- Mote context menus handle group selections
- Key control (ESC aborts any action, DEL removes mote(s)
- Several visual and behavior fixes (see commits)