Add mouse wheel / trackpad scroll event bindings to move through scanned pages

Simplescan is very nice. I especially like the 'reorder pages' feature (although I find its placement in the hamburger menu a little strange).

One little issue though: it seems that after scanning multiple pages (like 20 or so), it is not possible to scroll easily through the page view.

Perhaps mouse-wheel event binding could be added to allow scrolling (horizontally) through the collection of scanned pages? Something that also works with the trackpad, too, perhaps? The GNOME scroll bars are so hard to grab hold of these days, it really needs scroll wheel support for usability.