[merge] further progress bar fixes

[Martin Pool]

This fixes the bug in progress bars noted by John a while ago, and
does some further cleanups there.  There is more still to do.

One question that came to mind here is whether we really want a stack
of progress tasks, or a more general forest of trees.  You could
certainly draw the latter in a gui.  However, it seems to make the api
more complex, particularly if we want to get away from passing pb
parameters down the call stack, which would be nice.  Also, without
threads or some other asynchrony approach, I don't think we'll really
seriously have many unrelated tasks going at the same time.

This stack is redundantly held by the ProgressTasks and the ui at the
moment, and I'll fix that later.

I rename some of the ui_factory methods that are meant to be called
only from the ProgressTask to make them more private.

-- 
Martin <http://launchpad.net/~mbp/>