Update GIMP project meta information.

Make gimp.doap compliant to the new DOAP file style (GUADEC 2014):
* Add new mandatory fields 'description' and 'programming-language'.

Add download page URL.
Add bug-tracker URL.
Add programming language.
Set category to Apps.
Move Sven Neumann from maintainer list to AUTHORS after querying him.

Unify description in gimp.doap and gimp.appdata.xml.in.