Rewrite parts of segment linking

[robUx4]

At least the original medium linking section will need a bit of cleaning/rewriting. The text seemed clearer, in particular in relation to the relation with other kinds of linking.

This kind of linking is a mix between hard and soft linking. Each segment linked is independant from the other (standalone, unlike hard linked ones). But it should be treated as a hard-link by the player. Medium linking is done through chapters using the ChapterSegmentUID element and only makes sense for ordered editions. The Segment matching the UID should be played as if it was part of the original segment (segment it's linked from) and then resume playback in the original segment. That means the timecodes of the following content should be shifted by the duration of the linked segment. As for hard-linking, the resulting segment edition should be played and considered as one.