[1.8] Cinnamon Desklets

A system for users the create desklets (aka Desktop Widgets) that appear on the desktop.

Also requires muffin changes: linuxmint/muffin#29

Examples of known implementations:
Screenlets: https://upload.wikimedia.org/wikipedia/commons/0/0f/OpenSUSE_10.3_-_Screenlets.png
KDE Desktop Widgets: https://upload.wikimedia.org/wikipedia/commons/7/78/Contour-1.0-ActivityScreen.png

The desklet system is near completion. The only remaining task is to make it consitent with #1476

Currently in the process of creating more desklets
TODO
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- Create more desklets