Use a shorter format for observer metadata

Currently the rating/ping/cpu values shown in brakets to the right of an observer's name are generally not visible, in part because the box is too narrow.
This pull request does some trivial code cleanup (stop using globals everywhere!), factorises the obsever-message-building code, and has it employ a more concise format.

There exists an unrelated problem I hope to solve which casues rating values for observers to routinely show as "nil". Is this in the issue tracker? I'm as yet to come up with clear STR: testing is slightly awkward at the moment.