Refactor|World|Client: Private data for Polyobjs, revised ClPolyMover

Removed the underscored members of polyobj_s, as they were clearly
intended to be engine-internal. Now they have a better home in the
polyobj_s's private data (PolyobjData).

This cleans up the code nicely as one doesn't have to keep members
stored in void pointers, but instead they can be regular C++ member
variables.

ClPolyMover was updated to use thinker private data (like ClPlaneMover).
The related code in Map was removed.

Todo for future: Further cleanup could be achieved by treating polyobjs
as regular thinkers (cf. mobjs), removing the need for special handling.