clip: re-handle child in-point and max-duration

clip: re-handle child in-point and max-duration

The in-point of a clip is kept in sync with its core children, unless they
have no has-internal-source.

The max-duration is defined as the minimum max-duration amongst the
clip's core children. If it is set to a new value, this sets the
max-duration of its core children to the same value if they have
has-internal-source set as TRUE.

Non-core children (such as effects on a source clip) do not influence
these values.

As part of this, we no longer track in-point in GESContainer. Unlike start
and duration, the in-point of a timeline element does not refer to its
extent in the timeline. As such, it has little meaning for most
collections of timeline-elements, in particular GESGroups. As such, there
is no generic way to relate the in-point of a container to its children.