Support rotation of cropped stacks in cropping backend
Allow ROIs (and therefore cropped stacks) to be rotated

The ROI tool now has an additional box for the clockwise rotation angle
of the box.
Cropping tool: reduce slider size to 2/3 of default size

There are meanwhile quite some controls on the cropping box' toolbar. To
save some space, all three sliders have been made smaller to 2/3 of the
default size.
Merge pull request #531 from tomka/rotation-for-rois

Rotation for ROI tool and cropping backend