Add ability to scroll image using track-pad

At the moment it seems that the double finger up/down on the tackpad produces a zoom function. As far as I can tell there is no similar gesture for scrolling the zoomed image. 

This is particularly useful for laying down boxes for light pollution removal.

In most Mac software (I don't know if this is just a Mac convention) you double finger slide to scroll and pinch to zoom (this is how Photoshop does it and most other software packages). 

Note also that double-finger scrolling is also how all the other windows in APP are controlled (such as the console and the frame table) so it would be more consistent to use this gesture in the image window too.