Require schema prefix everywhere

There are various places in Nuxeo where we allow referring to properties without the schema's prefix, either for compatibility with old things or because the schema is configured without a prefix. On the other hand there are also places where we allow skipping the schema prefix even when it's configured.

This kind of leniency is bad and inconsistent and we should use and require schemas everywhere, which would clarify and simplify a lot some parts of the code.

We need to be backward compatible with user code for a while, so there should be a flag, on by default, to allow old usage.

One issue is that of storage, because DBS unfortunately uses the unprefixed form for schemas without prefix ("icon", "content"). This will need a database-level migration step.