Distinctive icon for labels

Labels so far use the 'computer' icon. Now they have their own icon:
http://rrze-icon-set.berlios.de/tango/gallery.html#attribute

The RRZE Icon Set is based on the Tango Icons Jenkins already uses. The 
license is the same CC-BY-SA as Tango, and I tried to reproduce the license 
reference in Jenkins as close to Tango's as possible; and added an 
additional link to the web site, as RRZE Icons are less well known.

I'm not sure whether all of these new files are required, but decided to
just provide the icon in all sizes.