Fix the background color of some messages
Improve XEP-0308 support

- Prevent correction of delayed messages
- Prevent correction of messages by someone else in a MUC (and in a
  private tab)
- Messages with unauthorized corrections (above) or wrong message id
  will be displayed as normal messages

TODO: restrict the corrections to the same fullJID (only in direct
"normal" conversations, because we can know in private an muc tabs, via
the User object)
Fix signal handling

- reload the config/theme with SIGUSR1
- quit properly with SIGHUP/SIGTERM
Fix a traceback due to a sleekxmpp API change

- remove the help message for people still using the old custom
    sleekxmpp repo
Remove a debug that floods too much