Removed adding a rect via a right mouse button click. restricted the selectable area to the area where the preview image is displayed.
When calculating the actual area, the preview's offset must also be taken into account, of course.
Thanks to Mr. kripton, we can now drag, drop and resize areas (cf. http://qt-project.org/forums/viewthread/24104 )
Store the actual area position inside the area widgets for now. Remap them when the geometry of the preview image changes.
Rotate all areas when the image is rotated.
Show how to call ImagePreview::areaPreviewToActual().