Add support for navigating between images when zoomed in using touchscreen/stylus

This is modelled after common mobile gallery apps, and works like this: You can zoom in and drag to a horizontal edge of the image. Then lift your finger (or stylus), grab again and drag over the edge. If you stay on the same image, you can navigate around in it, and to navigate, repeat the procedure. It is enabled for finger and stylus, if you have a mouse, click the < or > button. Possibly this should only be activated on mobile platforms - after all, the buttons are there on pc's.