Added trapMouseCursor() and freeMouseCursor() to sf::Window.
Reused existing Window Messages and updated clipping region to client rect rather than window rect.
Moved cursor clipping into a single method setCursorClipped() and fixed resizing events not updating the clipping properly.
Added setCursorClipped() for X11 and fixed some minor issues.

Also disabled the function for fullscreen windows and added some missing
initializer list entries.
Forgot to reset/reapply cursor clipping on focus changes.
And another tiny bug...