Avoid redundant key re-grabbing

XFCE desktop shortcuts are unusable for about 30-40 seconds after switching from tty to Xorg if there exists a .Xmodmap file in $HOME. Without a .Xmodmap file, the delay is about 10 seconds. Similar behavior gets triggered when a keyboard is reconnected to an USB port. With this patch, desktop shortcuts are usable immediately (likely in less than 0.25 seconds after switching from tty to Xorg).

An issue not fixed by this patch is that /usr/bin/xmodmap can be executed about 30 times after switching to Xorg. However, on the tested machine with a relatively small .Xmodmap this does not pose a performance issue which would be noticeable by the user.

Closes: #9 (closed)

Closes: xfce4-settings#168 (closed)

Closes: xfce4-settings#228 (closed)

Mitigates: xfce4-settings#167 (closed)

See also: xfce4-settings#93 (closed)