pageList and other EPUB collections must be serialized as JSON "compact collections"

[mickael-menu]

All the subcollections defined in the EPUB RWPM profile are supposed to be Compact Collections:

Compact Collection
A collection that contains one or more links, but doesn't contain any metadata or sub-collections.

But the current Kotlin toolkit serializes all subcollections as Full Collections, with an empty metadata object.

We can fix this by:

1. Adding an PublicationCollection.isCompact property, to determine if a collection must be serialized as a simple list of Link objects (compact collection).
2. When parsing a RWPM JSON, determine the value of isCompact automatically according to the parsed structure.
3. When creating an in-memory model from the EPUB parser, set explicitly the isCompact property to true, when creating the subcollections.