Thresholding applet extension (graph-cut)

This provides the thresholding applet with a new tab with graph-cut segmentation. For users that don't have opengm installed, the new tab will be greyed out. Graph-cut segmentation is ~~currently only~~ supported *object-wise*, i.e. 
  - the input data is thresholded in the regular way (`out = in>t`)
  - connected components and their bounding boxes are computed
  - the bounding boxes are extended by a hardcoded margin
  - sub-images corresponding to the bounding boxes are fed into the graph-cut algorithm

or for the whole volume (note the radio buttons).

Missing:
  - time slice handling