Extend support for behaviors

I'm currently working on implementing a tree field as an Entity Reference behavior, and bumped into several limitations.

I would like to make the following changes:

• Allow (field-level) behaviors to alter the field schema
• Allow (field and instance-level) behaviors to alter the property information
• Pass the full-fledged $field, $instance to the access method

The last modification should be possible now that we fixed the AJAX integration in the field and instance settings form: $field should now reflect the unsaved values of the field when an AJAX refresh is performed.