Add model fields for the loading state of a piece of content

This way views can subscribe to events like whether a piece of content
has loaded or failed to load
Show a progress bar when loading a piece of content
Do not wait until a piece of content has loaded before

changing the views
Show the title of a collection and a description in the book editor
Merge branch 'atc-only' into epub-only
The model promise waits for the navigation file to load