pqProgressWidget now renders the progress + label using an internal
class rather than using pqProgressBar (which has now been removed).
We don't directly use QProgressBar since we
want to the label to be painted on OsX and also control the appearance
of the widget a little more carefully.

pqProgressWidget uses fusion/cleanlooks on OsX to ensures that the
progress bar has label text on all platforms consistently.

Cleaed up pqProgressManager to avoid purging of progress events.
vtkPVProgressHandler already ensures that the UI does not receive
progress events too frequently.