Try to call desktopRezie once intead of doing it in all XRandR events.

When a screen is disconnected XRandR emits 3 events instead of just one:
1-Disconnected screen resized
2-Disconnected screen moved
3-Disconnected screen removed

Before this commit we were calling desktopResize on each event which
between other things restart the composite (not necessarily the faster
thing to do...).

So, in order to be able to call desktopResize only once, now we're
handling individually each event, when one of them happens a QTimer is
started/restarted on each event and desktopResized is called owhen
QTimer.timeout

The current interval is 100msec