gst: Add new flags for playbin

See https://bugzilla.gnome.org/show_bug.cgi?id=679031

https://bugzilla.gnome.org/show_bug.cgi?id=728020
backend: Use new "audio-filter" property

So that the audio-filter can be bypassed if it's not going to
make any changes.

https://bugzilla.gnome.org/show_bug.cgi?id=728020
all: Remove other users of video-utils.h

As started in 5b8498d98e59d57de0c573231056bac8504277b1