Segmentation Editor: Scrollbar reacts properly to PgUp/PgDown.
Segmentation Editor: Allows jumping to next/previous ROI.

Pressing the ,/. keys now jumps to the previous/next slice whith a
non-null selection.  Very useful for checking ROIs before doing
interpolation.
Merge branch 'master' into segm_editor_fixes
Merge pull request #18 from quazgar/segm_editor_fixes

Segmentation editor: Fixes and ROI searching