Only allow properties inside objects, this makes it easier to
iterate the object, which is needed for efficiently processing
control streams.
Add a choice type to mark variable properties.
SPA_TYPE_Enum -> SPA_TYPE_Id to avoid confusion with choice enum
Make it easier to allocate and initialize properties on the stack
Make more efficient methods to make objects.
SPA_ID_* -> SPA_TYPE_*. We use 'type' for all registered types.
ID refers to either a registered type or an enum.
Improve introspection of object ids. Make 0 an invalid enum and
use it to look up the object id type.
Move some type-info files together.
Swap type and id of the object, we first specify the type and then
the id because the meaning of the id depends on the object type.
