Handle enabling/disabling compositing while Guake is running

Currently Guake checks if compositing is enabled only at startup.
This causes problems in XFCE: Guake window is opaque when it's
started automatically on login. I have to restart it to enable
transparency. Also, it is generally better to react to changes
without restart.
Merge branch 'master' of https://github.com/sanya-m/guake into sanya-m-master