Dont run zeitgeist on GNOME

Zeitgeist is not really useful under Ubuntu GNOME, and in fact we don't ship the binaries by default, since we use tracker. However if a user has both Unity and GNOME installed, then zeitgeist will run under the GNOME session also. Presumably this is pretty harmless but will eat up RAM and I/O cycles so would be good to avoid it.

Hopefully just requires an AutostartCondition in desktop file(s) to fix.

ProblemType: BugDistroRelease: Ubuntu 16.04
Package: zeitgeist 0.9.16-0ubuntu3~gcc5.1
ProcVersionSignature: Ubuntu 4.3.0-2.11-generic 4.3.0
Uname: Linux 4.3.0-2-generic x86_64
NonfreeKernelModules: nvidia
ApportVersion: 2.19.3-0ubuntu2
Architecture: amd64
CurrentDesktop: GNOME
Date: Thu Dec 17 12:39:45 2015
InstallationDate: Installed on 2012-09-23 (1180 days ago)
InstallationMedia: Ubuntu GNOME Remix 12.10 "Quantal Quetzal" - Alpha amd64(20120922)
PackageArchitecture: allSourcePackage: zeitgeist
UpgradeStatus: No upgrade log present (probably fresh install)