Allow layouts to be incoked with fake data

This patch allow 2 things to be done:

 * Write unit test to validate layouts using fake clients and tags
 * Query the current layout geometry from another tag

The advantages of the former are clear and simple. Those for the later
include:

 * Creating screenshot of an other layout
 * Display the layout wireframe in the taglist (like KDE2-3, Gnome2)
 * Having and 'alt-tab' like visula popup for tags

Proof of concept:

![blender217](https://cloud.githubusercontent.com/assets/340384/4516506/2b41461c-4bfa-11e4-9c96-94f74b478fcf.png)