This new MotionManager handles movement for UnitMotion components (not
UnitMotionFlying).
This is a first step towards unit pushing, by giving a central place for
the relevant units to collide.
One important side-effect is that movement is effectively synchronous -
the positions are not actually updated until all units have moved for a
turn (refs 6a66fb8205).
As a side-effect, it's an optimisation: fewer messages are being sent
overall, which leads to a slight speedup (negligible without a lot of
units though).
This is a first step - ideally, the movement functions called from
UnitMotionManager would actually be moved there.
Differential Revision: https://code.wildfiregames.com/D3509
This was SVN commit r25071.
