overlay: Make cLock widget centered on default workspace

screenshot https://dl.dropboxusercontent.com/u/5779366/Screenshot_2013-04-12-22-21-14.png

* patch set 1:Initial commit
* patch set 2:Follow Trebuchet changes
* patch set 3-4:Fix commit message

Change-Id: I70cc193fb2c930b9d505525920a7a3533a41624e