RosterTooltip: Improve string formatting
This applies ellipsizing on strings for name, jid, and status messages. Overly long group chat topics (=status) will be shortened as well.
This will also shorten status messages correctly if there is more than one resource. Additionally, a new line is always added for the status message to keep the tooltip's width small.
Furthermore, I added a tooltip for the 'Observers' group to describe why contacts are grouped there.