Mouse wheel Z-frame scrolling

[dominikl]

Fixes Ticket 12829

Test: Open an image (not a big image) with multiple Z frames. Make sure the mouse wheel scrolling through the Z frames works.

[mtbc]

Good to merge.

[will-moore]

@dominikl Certainly the mouse-wheel scrolling works, but it sometimes also zooms the image. I'm not sure exactly what triggers this, but if I scroll up and down repeatedly, I usually end up at a higher zoom level. This happens most reliably at max Z when all scrolling results in zoom, but also can happen at anyt other Z-index too. Also, clicking and dragging of the image with the mouse still changes the z-index instead of panning, but maybe you meant to fix that in a different PR?

[mtbc]

Gah, sorry I missed that misfeature.

[dominikl]

@will-moore I can't reproduce the mixed scroll/zoom behaviour. There's a difference between small and big images. Small images: Scroll and drag change Z frame, no zoom; big images: scroll zooms in/out, drag moves the image around. I think that's intended.