Make mouse acceleration and sensitivity OS-controlled in menus

[beholdnec]

This patch changes mouse behavior in the following ways:

• Cursor acceleration and sensitivity are now controlled by the OS in the main menu, briefing screen, PDA and escape menu.
• When TFE is in windowed mode, the cursor can now be dragged outside the window.

I recommend merging with flattening for a clean commit history.

The following known issue occurs:

• The cursor should warp to the center when activating a menu (partially implemented but doesn't always work)

[beholdnec]

This should be ready for testing now.