Make it possible not to call URL pattern bindings setters or getters

URL pattern bindings are used as is:

• when restoring context for a given URL, setters will be called
• when building a document view before redirect, getters will be called.

We need to make it possible to disable a binding in one of these cases.
This is interesting for instance for compatibility bindings, where we want restful URLs using a deprecated binding to be called on set, but we do not want the corresponding binding name to pollute the generated URL on get.