plugin XEP-0384: update to latest python-omemo + better trust management:

- python-omemo minimal version is now 0.11.0
- `undecided` devices are now handled, fixing the "accept all or none" issue: if a device
  is `undecided`, the trust manager will be shown with the missing devices before sending
  the message.
- `untrusted` devices are discarded

setup.py

@@ -48,7 +48,7 @@ install_requires = [
     'urwid >= 1.2.0',
     'urwid-satext >= 0.7.0a2',
     'wokkel >= 0.7.1',
-    'omemo',
+    'omemo >= 0.11.0',
     'omemo-backend-signal',
 ]