[Icon Plasmoid] Provide a generic plasmoid class for icon based plasmoid

[Anne-Marie Mahfouf]

Hi all, hi Loic,

Tested Show Dashboard with trimmed code, works as expected. I would comment 
setAspectRatioMode(Plasma::Square); (as Marco said)
as it expands too much in the panel with this line (try making your panel half 
your screen with and without this line).
Also I would associate the "cashew" plasma icon for this applet, as it's the 
one that shows the dashboard from the desktop.

OK to commit?

Anne-Marie

PS: the icon code (Loic tarball) should go as a test somewhere, would 
kdeplasmoids/applets/tests be OK?

Loic, can you start a new thread for next patch please