Allow rebinding mouse buttons/scrollwheel

[DarkDefender]

I'm currently using a very hacky patch to rebind the scroll wheel to move:

diff --git a/src/imv.c b/src/imv.c
index 24054da..5631c70 100644
--- a/src/imv.c
+++ b/src/imv.c
@@ -1151,7 +1151,8 @@ static void handle_event(struct imv *imv, SDL_Event *event)
       }
       break;
     case SDL_MOUSEWHEEL:
-      imv_viewport_zoom(imv->view, imv->image, IMV_ZOOM_MOUSE, event->wheel.y);
+      //imv_viewport_zoom(imv->view, imv->image, IMV_ZOOM_MOUSE, event->wheel.y);
+      imv_viewport_move(imv->view, event->wheel.x * 20, event->wheel.y * 20, imv->image);
       SDL_ShowCursor(SDL_ENABLE);
       break;
     case SDL_MOUSEMOTION:

However this is not a good solution. I think the best would be to be able to rebind mouse events to different actions.

[eXeC64]

Hmm, given how few ways people will probably want to use scroll events and how important getting the right feel for them is, I think this should probably be a couple of config options rather than binding discrete actions to a command.

e.g.

[options]
mouse_scroll_action = <pan|zoom>
mouse_scroll_speed = <floating point value controlling zoom/pan speed.>
mouse_scroll_reverse_y = <bool>

Does that seem reasonable, or is being able to bind scroll_down to executing a command really important? I think I'd prefer to have really smooth natural zooming/panning rather than binding scroll to discrete actions.

[DarkDefender]

This does seem more reasonable to me! :)

I also think people would prefer smooth scrolling over binding the events to operators.

[Arnaudv6]

probably more related to #211, but can you please consider
prev/next image on mouse wheel?