PersistentState service + full integration with ViewEditor

[judicaelclair]

• PersistentState manages data that has a lifetime longer than the program (dumped to a config file).
• PersistentState currently stores colours & views (1-6) that a user can switch between
• Periodic and immediate saving of state
• Graphics refactored to use it - specifically VoxelMap/MapData and ViewEditor rework.
• Added capability for an event to be _unifiable. Such that if the event is posted multiple times before being handled (popped off the queue), the existing event currently in the queue will absorb the event to be posted. Particularly useful for KeyFrameEvent that is often spam posted dozens of times before it is actually handled. Handling a KeyFrameEvent is computationally expensive, which means having a method for discarding redundant posts is key.
• Support for dynamically changing algorithm-specific colours with persistence. Currently tested on A* & RRT.

[rm1819]

PersistentState ready to merge to master