winemac: Send WM_{ENTER, EXIT}SIZEMOVE before/after window dragging and run an internal event loop during.

This simulates some of what would happen if user32 were managing the drag.  The
click in the caption would cause WM_SYSCOMMAND/SC_MOVE.  The processing of that
message is synchronous and doesn't return until the move is complete.

Some games require that "blocking" in the internal event loop to prevent them
from misbehaving during the drag.