Smarter ordering of nicknames for tab completion

[pulkomandy]

On busy channels, there can be many people with similar nicknames. When talking with one of them, it is not uncommon to accidentally tab-complete to some other name with the same prefix.

Avoiding this requires making the tab completion handler 'smarter':

• Memorize the last nickname that was tab-completed. If the prefix in a new tab-completion request matches, we can suggest that as the first completion
• For the remaining users in the list, instead of going through them alphabetically, we can order by who sent a message most recently. This way it is easy to reply to someone who recently spoke (which is more likely to happen than seinding a message to some other person, and also you are more likely to be less careful about if the tab completion worked as expected in that case).