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

When moving around many motes in the visualizer frame, it may cause headache having to select and move every single mote.

Thus this commit adds user-friendly 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

Readio medium visualizers are adapted to show adequate information for
multi-selections.

Note: This changes previous behaviour of using Ctrl key.

As this change required to change major parts of the visualizer input handling and the radio medium paintings, it will be best to test and see how it works instead of walking through every single loc ;)