This is more in line with wayland and it allows us to create new
interfaces in modules without having to add anything to the type
enum. It also removes some lookups to map type_id to readable
name in debug.
Don't pass pw_properties around when we simply need to pass around
config info, only use pw_properties when used to construct an object
that keeps the properties.
The proxy API is the one that we would like to expose for applications
and the other API is used internally when implementing modules or
factories.
The current pw_core object is really a context for all objects so
name it that way. It also makes it possible to rename pw_core_proxy
to pw_proxy later.
Metadata allows apps to attach properties to objects that can be
read by other apps.
Not complete yet, properties should be removed when the object is
removed.
