Rewrite mouse handling code (incomplete)

Fixes #1300, #1556, #2081, #3284
MainFrm: remove unused variable
D3DFS: remove horizontal margin from osd posbar

The bug with autohiding is now fixed and there's no reason to have it
there
Call OnIdle() less frequently
ChildView: remove unneeded critical section
Mouse: madVR compatibility hack
PlayerNavigationDialog: minor refactoring