backend: Use GDK_EVENT_PROPAGATE/_STOP constants

instead of TRUE/FALSE.
backend: style fix
main: Remove drag support from the video widget

I doubt anybody ever used that, and it conflicts with gestures,
so dump it.
backend: Use ClutterTapAction to show/hide the controls

https://bugzilla.gnome.org/show_bug.cgi?id=697896
main: Handle scroll events inside the video widget

And pass the "requests" to the player itself. That will allow
us to use gestures to control those in the future.
backend: Don't allow seeking if not seekable

https://bugzilla.gnome.org/show_bug.cgi?id=723981