Add a setting to disable picture cycling on scroll

I implemented a new gconf setting to prevent the image view from cycling the images on scrollwheel input as requested in issue #34 (closed).

By default it keeps the current behavior. If you toggle the setting, it disables pictures cycling and enables vertical scrolling.

I tried to keep the merge request as simple as possible for you to review, so I did not implement a switch in the preferences window (I can do it if you want).

Please tell me if I should do anything else