[cooja] plugins/Visualizer: Multi-mote selection functionality

This adds multi-mote selection capabilities to the visualizer plugin:

- [Ctrl + Mouse Drag]: Rectangular selection of multiple motes
- [Ctrl + Mouse Click]: Add/Remove motes from current selection
- Mouse Drag on any selected Mote: Move all currently selected motes

Note: This changes previous behaviour of using Ctrl key.
[cooja] plugins/Visualizer: Updated quick help to match new control
capabilities
[cooja] plugins/skins: Adapted DGRM and UDGM visualizer skins to show
adequate information for multi-selections
[cooja] apps/mrm: Adapted MRM visualizer skin to show adequate
information for multi-selections
[cooja] Added @Override annotations for Visualizer and skins
[cooja] Automated code updates that do not make any functional changes
- made loggers final
- added some diamond interface cosmetic
- shortened if-return statements
- replaced size == 0 check with .isEmpty()
[cooja] plugins/Visualizer: Use more specific catches and multi-catches
if possible
[cooja] Reformatted Visualizer and skins code
[cooja] plugins/Visualizer: Replaced multiple if-string with switch over
strings