pipeline: add a get_mode method.
ges-audio-track: Change contructor prototype.

We return an AudioTrack.
timeline: when there are no objects anymore, set duration to 0.
container: resort children after prepending an element.
trackelement: update control bindings correctly.

When duration or inpoint change, we need to remove edge control points,
and set new control points with interpolated values.
Also when duration == 0, we need to remove all control points, as otherwise
the controller will raise !is_end assertions.
It's the duty of the application to set keyframes back when duration gets
!= 0 again.
trackelement: split bindings correctly.
assets: add all effects assets at initialization.
integration: add an argument allowing to test specific effects.