Add progress monitor support to processors

It should be possible to observe a processors inner state, i.e. when it is beginning processing data, when it finishes and the progress in between. This would allow to add a progress bar to the GUI that displays it. This would also nicely be integrated with ITK filters that can be monitored via observed->AddObserver(observer, itk::ProgressEvent). Another consideration is if one processors calls other processeors internally, then their progress should be added to the parent's progress.