A widget specifically layouted for the status bar can now be registered
with the progress information. The newest one is made visible next to
the summary progress bar. If a newer one vanishes, the older becomes
visible again.

Change-Id: Iedf0e88a542ea442ae86fa51c792c68fbc6eef3c
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>
Reviewed-by: Eike Ziller <eike.ziller@digia.com>