Added complete API for zoom/unzoom (scale and scroll) and
rotate/unrotate, with the same set of functions as the existing
transform/untransform. Moved some special case functions to the
namespaces they belong.
First version of display rotation, inspired by gimp-painter.
The rotation always happens around the image's center.
The only "UI" for rotating is currently shift+middle-drag and
shift+space-drag. Control constrains the angle to 15 degrees
and is currently the only way to go back to "no rotation".