Rotation for ROI tool and cropping backend

The commits in this branch add optional rotation to the ROI tool (which is basically used by the cropping tool at the moment) and the cropping backend. A new text box allows to add an angle in degrees. To save some space on the cropping toolbar, the size of the sliders in the toolbar has been reduced to 2/3 of the default size.

![catmaid_rotation_for_rois](https://f.cloud.github.com/assets/112226/507297/79602e1c-bd6c-11e2-884f-e61973b6e0db.png)