Cropping tool: comment changes
Cropping tool: use jQuery-iterator to loop over stacks
Add ROI tool base class and let cropping tool use it

This commit introduces a tool which can be built on when a one needs to
support box selections in the current view. The tool provieds toolbar
fields for position and size manipulation. This hab been factored out of
the cropping tool to allow re-use of this part.
ROI tool: show '-' instead of 'NaN' for NaN-values in text boxes
Cropping tool: remove added elements when tool is destroyed
Merge branch 'roi-tool'