ges-source-clip: fixed return of duration setter
In general, brought the behaviour of the `start`, `duration` and `inpoint` setters in line with each other. In particular: 1. fixed return value the GESSourceClip `duration` setter 2. changed the GESClip `start` setter 3. fixed the inpoint callback for GESContainer 4. changed the type of `res` in GESTimelineElement to be gint to emphasise that the GES library is using the hack that a return of -1 from klass->set_duration means no notify signal should be sent out. Also added a new test for clips to ensure that the setters work for clips within and outside of timelines, and that the `start`, `inpoint` and `duration` of a clip will match its children.
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