Viewport: Only zoom in/out at vertical wheel events

Viewport always zoomed the signals in and out on a wheel event. Moving the
scroll wheel up or down caused the intended effect of zooming in or zooming out,
respectively. This works flawlessly on mice with only a vertical wheel.

On mice with a horizontal or tilting wheel, horizontal scrolling or tilting was
still treated as zooming in and out. This created a non-intuitive experience,
as the user expects the signals to scroll left or right with this gesture.

Check to make sure the wheel event is a vertical one, and only then zoom in/out.
Do nothing if the wheel event is not vertical.

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>