Webkit feature parity

[JacksonRG]

1. When the window is resized, parts of the ruler that weren't visible had no ruler marks. Now, changes in width attribute the ruler's parent container should trigger redrawing. (Thought that was fixed in my last PR)
2. Horizontal scrolling hasn't worked for me when running with the webkit backend. Now both web backends should respond to shift+mousewheel as horizontal scrolling

[ferdnyc]

The shifted horizontal scrolling does work for me now, but it's very jerky on Linux and keeps trying to scroll vertically at times, especially when I reverse direction while holding down Shift. I suspect a .preventDefault() is needed.

[ferdnyc]

Oh, nice, and tilt-wheel horizontal scrolling still works as well.