Implement desktop update notification hacks

[azubieta]

Right now we don't call any desktop update from libappimage, first we need to check if we want to do it. Second we should move all the known hacks (not only kde) to libappimage

Originally posted by @azubieta in #123 (comment)

[TheAssassin]

That's part of TheAssassin/AppImageLauncher#165 IMO. We should first sort out all remaining libappimage issues and make sure it's reliable enough to use it in AppImageLauncher. Once that is possible, we can discuss moving functionality around.