Revamped XML for the last time, and added ability to save XML.
Added validation, but only for systems with the lxml module installed.
Added color to all simobjects
Added color parsing to the XMLParser and simulator
Added color to the interface
Tabified robot docks by default
Nicer management of dock windows
QuadTree for rapid collision detection and sensor updates
Performance improvements to QuadTree class
Removed redundant code from simulator
Merged with physics branch
dock expand on title click
Keep dock positions on rewind
Merge branch 'rainbow_robots' into stage
Adjusted zoom
Switched to RelaxNG validation, because it's way better.
Fixed unit tests for the new XML parsing code somebody else added.
Updated simulation.rng.