StatusBarManager: Simplify code

We can simply qobject_cast, since everyone is simply adding their
statusbarwidget to the object pool and not a aggregate.

Change-Id: I838a36ef532f3c68c9c41d71b69edadaa1d278de
Reviewed-by: Eike Ziller <eike.ziller@digia.com>
ImageViewerPlugin: Remove useless code

Change-Id: I4e475df102638d5c8895b14b8bdecfacd94a565b
Reviewed-by: Eike Ziller <eike.ziller@digia.com>
ModeManager: Simplify ::objectAdded/objectRemoved

No-one is using the aggregated IMode, everyone is deriving from IMode

Change-Id: Ic4f21ce983924afe80cc2180101aae838b9f590d
Reviewed-by: Eike Ziller <eike.ziller@digia.com>