Make wmaker XRandR aware

Make wmaker XRandR aware

This patch makes wmaker automatically Restart() to take into account
resolution changes done by xrandr.

The motivation to write this patch is that when I start my laptop connected
to an external monitor (VGA1), the resolution in the monitor is initially
wrong (perhaps this is a bug in XOrg or OpenSuse, but anyway), so I always
do this after starting wmaker:

xrandr --output VGA1 --preferred

The resolution in the external monitor gets fixed, but wmaker does not
recognize the change automatically and I have to manually restart it.

So this patch avoids the last step by making wmaker be XRandR-aware.
Wmaker now restarts automatically when it receives a RRChangeScreenNotify
event and I don't have to do the last part of the silly procedure described
above.

I am not sure if restarting wmaker is the most elegant solution, but
it at least solves part of my issue with wmaker + xrandr.