Move away from navigator’s internal methods/properties

[JayPanoz]

Right now we’re using internals to inject scroll affordances (nav._cframes, which is temporarily exposed for lack of APIs but will be made private in the future), but also to inject ReadiumCSS custom properties (see #17).

It should be made clear people should not rely on these private members and the Playground codebase moves away from them as soon as possible.

(As exhaustive as possible) listing of these private members:

• _cframes
• frameManager & frameManager.msg.send
• pool & pool.setPerPage()
• setReadingProgression()

If possible, developers/implementers of the TS-Toolkit Navigator should not heavily rely on these, and issue a feature request if they need something that is not covered through Injection (injecting into iframe’s content document) and Preferences (user settings) APIs.

In case of doubt, members that are exposed but will become private should be indicated in ts-toolkit src.