Added ability to grab the cursor

 * When grabbed, the mouse cursor may not be moved outside a window's
client frame.
 * Fullscreen windows always grab the mouse cursor.
 * The effect is only active while the SFML window is the active
foreground window.
 * Right now this is only implemented for Windows and Unix.

Signed-off-by: Marco Antognini <antognini.marco@gmail.com>
Added iOS and Android 'not applicable' implementation of setMouseCursorGrabbed
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.
Renamed the two wrongly renamed grabCursor to ClipCursor.
Merge branch 'feature/grab_mouse'

Conflicts:
	src/SFML/Window/OSX/SFOpenGLView.h
	src/SFML/Window/OSX/SFOpenGLView.mm