add transparency background function for workspace-switcher

this function set the the background of the applet to transparency if an user choose a transparency panel.
some examples:
![Bildschirmfoto](https://f.cloud.github.com/assets/961604/214044/da1c1adc-83e6-11e2-80c2-219d4ac97cf8.png)
![workspace-switcher](https://f.cloud.github.com/assets/961604/214070/78c3233a-83ea-11e2-98c8-c640ec357efc.png)
Hope you see the little different :)
Edit:
a screenshot with a real big panel, here you see the transparency in the space between the workspaces.
https://dl.dropbox.com/u/49862637/Mate-desktop/Bugs/work-space-switcher2.png