ENH: use the optimized function to initialize seeds

from the label image (vtkPichonFastMarching::addSeedsFromLabel() added in Slicer trunk r21701).
ENH: some minor stylistic cleanups
ENH: disable volume slider by default; check the number of seeds in the initial label volume before running the filter
ENH: move processing routines from tool to logic
ENH: add undo functionality
ENH: initialize volume scroller to [0,FM npoints]
ENH: shift the input intensities to not have negative values
ENH: screenshots illustrations (to use later for documentations etc)
ENH: Merge of Andrey's master branch