New ‘auto-correction-mode’ for the Free Rotation image plugin

A few weeks ago I added a new feature to the Free-Rotation imageplugin: 'Automatic (Horizon) Correction'.

This is nothing new, Gimp and Photoshop (and for sure other applications) have such a feature, too.
You set two points in the preview widget and hit "Adjust" to rotate the image accordingly to these markers.

The difference is that it is suitable to adjust horizontal and vertical lines and determines automatically which of the modes it should use, based on
the angle between the two marker points. Also the rotation direction is detected by the plugin, so that it is actually quite easy to use, you do not have to
say if you want to rotate clockwise or counter-clockwise.

The user interface could be better, for example it would be nice to have zooming capabilities for better marker control. But right now it is not possible with the
current preview widget implementation (we are working on it... :-)).