Make StyledMessage more (space) efficient, clean up ChatlineModelItem

* We really don't need to store styled strings for sender and timestamp
* We use only one format for all timestamps and one for all senders, no need to duplicate
* EditRole now provides raw undecorated data (i.e. QDateTime for timestamp and nickname for sender)
* QDateTime is shorter than a stringified timestamp
* No need to store constant strings (for sender col in non-plain msgs) in every StyledMessage
* Could simplify ChatlineModelItem as a result