Add support for views

[SanderMertens]

Views are objects that aggregate data from multiple objects with a declarative API. Each member in a view can potentially come from a different object.

A view shall subscribe for updates of the objects that it exposes information of, and update itself when one or more of its objects update. All fields of a view will be optional. When an object sends a DELETE notification, the associated members will be set to NotSet.

A view shall be able to contain collections that can aggregate elements from multiple objects. Collection types in a view will be limited to instances of the (unordered) list type.

Views will not rely on in-memory object references, but instead will use resource id's to refer to members. This way, a view can be used to store only a relevant subset of the data in memory, which is especially useful for large objects.

A view shall be agnostic to the logic that determines which objects it will subscribe for. For example, a query may rely on a view to make its data available and use query statements for joining objects. An application on the other hand could define a custom object-to-view mapping.