Refactor instance members for Updates class

Consolidates instance members for Updates class (_callbacks,
_got_finished, _got_signal, and _worker become a dict called _worker;
the old _worker becomes 'instance' in the dict).
Better user agents

Improves usability of this to push an update in the event of an odd
combination of things that causes problems.
Basic update dialog skeleton

This displays an intro line, a list of changes, and a synopsis at the
end. More to come.