ges-source-clip: fixed return of duration setter

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.