Currently the view supports histogramms only with one image and <=1 ROI.
Should also be flexibel for n images and m ROIs.
- Histogram widget needs to support IDs besides labels and histograms. Reason: Labels are constructed by node names and may not be unique. The IDs of historgrams should be constructed out of the IDs of image (and ROI).
- refine QmitkChartWidget (see sub task)
- adapt ImageStatisticsView to add all historgrams to the histogram widget. The widget should add, update charts or remove obsolete ones from the chart widget
- in addition HistogramWidget must be reworked to properly allow to be updated/changed.