Mouse-based interface for Cube: you left-click anywhere on the grid

Mouse-based interface for Cube: you left-click anywhere on the grid
and it moves the polyhedron in the general direction of the mouse
pointer. (I had this in my initial throwaway Python implementation
of this game, but never reimplemented it in this version. It's
harder with triangles, but not too much harder.)

Since the logical-to-physical coordinate mapping in Cube is
dynamically computed, this has involved an interface change which
touches all puzzles: make_move() is now passed a pointer to the
game_drawstate, which it may of course completely ignore if it
wishes.

[originally from svn r5877]