Mouse picking - physics

Just the underlying capability at this point.  Most of the features are missing (e.g. logical buttons, moving/rotating objects, etc).  However those features can (and should) be added as a separate exercise.

Please review before I get too far down (possibly) the wrong way.

Mouse clicks are disabled by default.  User setting under "debug" for mouse-picking needs to be enabled.  For now right mouse button is used to select the objects.  Have not yet implemented 'unique selection' i.e. selecting one object unselecting the previous object (but that should be simple).