Run GIMP in a D-Bus session

For instance see bug 683177: when trying to access the online help
users got an error message, that GIO backends were missing and they
needed to install GVFS. 
Actually the GIO backend GVFS is there, but had no D-Bus connection
(GVFS relies on D-Bus behind the scenes). 
This fix updates some packages and ensures that GIMP always runs in a 
D-Bus session and thus the online help is accessible.