Check page provider reference when registered through content view

Commits at NXP-15121 introduce a small issue in the model: page provider properties are supposed to be retrieved from the service API, not from the definition.

Properties are supposed to be referenced from the content view, and resolved from there (because they're in a JSF context), and then passed trhough API to the underlying page provider.

Commit at https://github.com/nuxeo/nuxeo-jsf/commit/af130b22036a0377294732c794cae199bf5d942f to review (i may have missed something)