[cooja] plugins/Visualizer: Fix: Always highlight selected motes.

Motes were highlighted only by the UDGMVisualizerSkin before.
Now highlighting moved from individual VisualizerSkin to Visualizer.

A selected mote is highlighted with a blue circle
and a semi-transparent gray overlay.
[cooja] plugins/Visualizer: Enable multi-mote deletion in mote menu
[cooja] plugins/Visualizer: Fixed node selection in Delete menu handling

Previously if multiple motes were selected
and the context menu of a non-selected mote
was used to delete, not the clicked mote
but the other selected ones were removed.

This behavior is fixed so that if another
mote is clicked only this is deleted.
[cooja] plugins/Visualizer: Implemented multi-mode behavior of delete mote menu for move, too

Note that this does not yet implement the correct behavior of beginMoveReques()
[cooja] plugins/Visualizer: Group mote placement modifications together (move, delete)
[cooja] plugins/Visualizer: Mouse release always terminates previous action
and reses actions state and cursor appearance

Also renamed UNKNOWN state to NONE as it is a more adequate name for the new behavior
[cooja] plugins/Visualizer: Partly implement behavior of beginMoveRequest to fit new mote movement handling
[cooja] plugins/Visualizer: Key control: Allow to abort actions with ESCAPE key and delete motes using DELETE key
[cooja] plugins/Visualizer: Deselect motes when removing

This also fixes visualization issues such as
remaining transmission range underlays for removed motes
Merge pull request #760 from ejoerns/pull-req/cooja-multi-mote-vis

[Cooja] (Multi-)Mote visualization updates