Removed unused otr callbacks
Moved otr autocompletion to function
Added contact autocompletion to /otr start <contact>
Open new chat win, or use existing for /otr start <contact>
Implemented /otr start <contact>
Check for already established but not used OTR session on /otr start <contact>

For example, one may have been established from the other end, without
any messages being sent yet
Fixed GString freeing before use in otr.c