Allow mouse middle scroll wheel button to apply panning action when image is zoomed in

When zooming in where size of image extends beyond the window size.
Currently, LMB is linked to panning action when held down and dragged.
Suggesting to link this same action to the middle scroll wheel button as well when held down and dragged.
This is a fairly common action linked to MSW button.
I would still keep linked to LMB because it is also convenient.

This request will make more sense to add to the program if the following request is also added
#134 (closed)

because if user can link zoom action to MSW then there finger would already be on the MSW when zooming in and out, which would make this MMB panning request more valuable to have as well.
These two requests I believe are common actions often linked to MSW and MSW button.

Thank You