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
Closes: xfce4-settings#168
Closes: xfce4-settings#228
Mitigates: xfce4-settings#167
See also: xfce4-settings#93



Signed-off-by: Jan Ziak <0xe2.0x9a.0x9b@gmail.com>