Add GPL exception to license.

The exception allows applets, workflows and plugins extending ilastik
through controlled interfaces to have non GPL compatible licenses.
Change license statements to mention GPL exception.
Add license dialog.
Display license from buildem when running from a binary install.
Binary distribution license path is different for windows.
Display-Thresholding added in LabelingGUI, depends on volumina changes
Display-Thresholding added in LabelingGUI, depends on volumina changes
Display-Thresholding added in LabelingGUI, depends on volumina changes
PixelClassification: show/hide the thresholding tool button depending on whether or not the data has multiple channels.
PixelClassification: Thresholding button is shown or hidden based on the layer type, not the number of channels.
Tooltip for Window Leveling button
CarvingDrawer: Added thresholding toolbutton
based on the grayscalelayer-flag window leveling it is determined if the window_leveling feature is active
split-body: Updated split-body tools for changes to the applet API and carving ui.
split-tool: serializer: Correct dirty handling of annotation filepath slot.
split-tool: Fix a non-int roi bug.
DataSelection: Minor gui tweak (resize table for columns)
VigraWatershedViewer: Avoid crash during shutdown by checking for sip.isdeleted
pixel classification: Adapted to operator changes in lazyflow scikit-classifier branch.
pixel classification: adapt to changes in the sklearn-classifiers lazyflow branch