Added a "positionable" property to the categories. Added a checkbox to the settings ui and added a respective property to the XML structure.
Reverted some unintentionally changed code lines to their original state.
Changed the "positionable" data type from int to bool.
Changed some more code lines so that the patch is minimal.
A QRect can now be selected on the annotation dialog's preview image. Also, a corresponding scaled QRect for the actual image is calculated.
Draw frame during selection, draw frame for final selection.

Warning: the way the final QFrame is created this is an evil memory
leak! Adjust this to suit your needs.