build: Require GStreamer core 1.2.4

To get this necessary bug fix, making "Make available for offline
use" work as expected:
https://bugzilla.gnome.org/show_bug.cgi?id=724373
main: Remove unused code

This code to capture events on the vbox that contains the video widget
isn't needed anymore.
backend: Avoid double-scroll events

We were receiving both the smooth scroll and the non-smooth scrolling.
As all the backends we support have support for smooth scrolling,
only ever handle those.

https://bugzilla.gnome.org/show_bug.cgi?id=728227