Enforce non-null types in the protocol

[Christopher James Halse Rogers]

These commits add a new, optional attribute to string, array, and objects
in the protocol: allow-null. If an argument is marked as allow-null=false
or does not have an allow-null attribute then attempting to pass null is
a protocol error, is picked up in libwayland, and the call is not made.

This makes it easier to write a reliable compositor (or client), as
implementations can now assume that objects passed to them are non-null
unless they've explicitly allowed them to be.