Switch to Preference API

[JayPanoz]

In the lack of Preference API in ts-toolkit, user preferences (and ReadiumCSS) have to use methods that are left exposed but will no longer be public once the Preference API is added.

For instance, frameManager.msg to inject ReadiumCSS properties into iframes’ html, or nav.pool.setPerPage to select the number of pages to display in FXL.

These should obviously be migrated once the Preference API is available.