Trapping/Clipping the Mouse Cursor

This implementation fixes the request discussed in issue #394 and is fully functional for Windows, but its still lacking its Linux/MacOS counterparts.

I picked the current naming scheme (`trapMouseCursor()` and `freeMouseCursor()`) instead of using some `setCursorTrapped()`, because it might be interesting to allow some custom rectangle to be passed as an optional parameter (overload?).

I'll try to create the X11 implementation myself, if noone volunteers, but I'll have to leave the MacOS one, because I've got no way to do or test this one.