Interactive Segment Tubes: Add drop seed button

This button controls the seed dropping. Since in the interface we don't see
the drop seeds icons anymore (the one in the toolbar), we need to add one
here specifically.
Interactive Segment Tubes: Drop seeds when running the segmenter
Merge branch 'InteractiveSegmentTubes_AddDropSeedsButton'

* InteractiveSegmentTubes_AddDropSeedsButton:
  Interactive Segment Tubes: Drop seeds when running the segmenter
  Interactive Segment Tubes: Add drop seed button