x11: Don't move X11 input focus during grabs
On X11, the display system's own input focus can undermine Clutter grabs, e.g. a newly activated window can take away focus from supposedly system-modal modes (shell overview, dialogs, ...).
Fix this, as well as an related issue where the tracking of the server focus xwindow messes up the stage's focused state.
Ah, isn't X fun ...