Fine-Grained Query initializers

[groue]

This pull request defines three distinct @Query initializers, that make it possible to fine tune who can update a tracked request:

• When it is the view that contains a @Query property wrapper, initialize @Query with an "initial request".
• When it is its enclosing view, initialize @Query with a "constant request".
• When it is both views, initialize @Query needs a request Binding.

The Adding Parameters to Queryable Types guide was updated with a detailed description of those various cases.