Add support for extra launcher dir
  
Hardcoding a single extra launcher path is impractical for distributions, as
multiple packages would collide on the name and wouldn't give any extra
customization possibility for OEMs.
          
Obsolete /usr/share/applications/indicator-session-extra.desktop and instead
read /usr/share/indicators/session/applications/*.desktop

https://launchpad.net/bugs/727823