Added OS X implementation of sf::Window::setCursorGrabbed
"Fixed" one corner case on OS X regarding cursor grabbing

Moving the cursor fast outside the window and clicking can sometimes
focus another application.

 * This is a HUGE workaround to keep the window focused when
   the user click outside the window by moving the cursor fast enough.
 * Even with this workaround, the window will lose focus for a very
   short moment, making it blink. Not really nice...
 * To be more complete, this workaround should have TWO monitors: one
   to watch for mouse events in other app (global monitor), another one
   to watch for events in other SFML window (local monitor).
 * I couldn't find a way to get the corresponding mouse up event so
   there is an unbalanced number of mouse down and mouse up events...
 * The user can still minimise the window (yellow button on the window
   bar) or move the window by clicking the window bar.