- 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