Improve horizontal scrolling behavior

[foghawk]

Currently, horizontal scroll-wheel events are handled oddly. The scroll wheel can never be used to scroll horizontally, even when a horizontal scrollbar is present (contradicting the behavior described in the scroll_wheel_event docstring), and both scroll protection and the flip with wheel preference are ignored if the page would be turned forwards.

This patch permits horizontal scrolling when applicable and respects protection (and therefore flip with wheel) in all directions. _scroll_with_flipping already handles manga mode, so there's no need for this logic in scroll_wheel_event.