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Else it could happen that we add the oob tag to a message
that gets later encrypted

This will only be added on not encrypted Messages

Show an error message when OMEMO is activated,
but python-cryptography is missing

- There is no need for discovery code, Gajim does this already
- Generate only one Base instance per account
- Use less global vars and simplify code

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.