Coding standards and documentation improvements in dispatcher_nb.py. Many open questions remain at my side. Please check FIXMEs.

Fix a potential bug of missing an alarm_callback. Separate public from internal API.

Thanks to omega for idea.

Now also highlighted for @nick or <nick> but not for "Gajim" if you are "jim"

revert [10848] That's not the way to solve the problem you're trying to fix: first keepalive packet is sent too early.