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