Make "skipad" action more generic

[martinthomson]

The "skipad" action is a fairly narrow action that represents a common thing that is shown on streaming services.

It might be useful to make this more generic, so that it can be used to skip any section of the media that is marked as being skippable. That applies to ads, intro sequences, and credits and maybe more.

In reviewing the feature, the TAG noted that this has a potential relationship with the chapter metadata. If a chapter were marked as "skippable" in the metadata, then a "skip" button could be shown. Also, the metadata might be able to provide localized text for that skip button so that the button could say "skip ad" or "skip intro" or whatever, as necessary.

Implementations might recognize "skipad" as an alias for this new action, or maybe just remove that given that some implementations do not presently support "skipad".

[youennf]

Discussed at TPAC 2024.
There was agreement to move to skip to support some other cases like chapterized video.
Marking this as ready for PR.