[Request] Mousewheel functionality change scroll/zoom/etc

Currently Ristretto uses the mouse wheel for changing the actively viewed image and Ctrl + wheel to zoom in and out. In my opinion this control scheme isn't optimal or standard as compared to other software (especially scrolling to advance the images doesn't work too well due to the slow image load issues #29 (closed) #16 (closed)). I think more options should be exposed for mouse wheel control.

Proposition:

• Preferences screen has a drop-down menu for what action the mouse wheel does with the following options:
  • Mouse wheel changes between images (current behavior)
  • Mouse wheel zooms in by default (same behavior as right now but without holding ctrl)
  • Mouse wheel pans image vertically
  • Mouse wheel pans image horizontally

I change between operating systems a lot as I need to use some system specific dev tools and ristretto has been causing me a bit of a headache when I forget to hold ctrl to zoom in and end up scrolling through images (often with minor freezes due to abovementioned issues). I think this change would be beneficial to many.