- Turn `SegmentList` into an only-child in `Period`, `AdaptationSet`
  and `Representation`
- Set `parent` type in `SegmentList`
- Turn `SegmentList.segments` from a property into a regular method,
  similar to `SegmentTemplate.segments()`

Inherit attributes from ancestor `SegmentList` nodes, if set,
similar to `SegmentTemplate`.
Also fix `presentationTimeOffset` not being inherited in
`SegmentTemplate`.
TODO: use common `MultipleSegmentBaseType` class for these attributes

The number of `SegmentURL` nodes in `SegmentList` can be zero, e.g. when
the `SegmentList` gets overridden in a descendant node

- Fix `SegmentTimeline` in `SegmentTemplate` and find ancestor timelines
- Add `SegmentTimeline` to `SegmentList` (currently unused)

Merged