Episode 2: no core changes required

 - An error during the upload now no longer hangs everything (GUI changes placed
   in main thread)
 - Some formatting changes
 - Update version in manifest (I think a new minor version is in order for GTK3
   support!)
 - Add self to authors in manifest (shameless plugs, whoop whoop)
 - Remove claim of supporting older versions of gajim

 - Move thumbnailing code into separate module
 - Port GdkPixbuf thumbnailer to gtk3
 - Make GdkPixbuf thumbnailer more efficient
 - Better exception logging
 - Some code reformatting

Not perfect yet, some layout issues remain, but it works.

added ability to do periodic plugin update checks (every 24 hours) and resized window to fit omemo plugin name properly

Fixes #60