SPA_MEMBER is misleading, all we're doing here is pointer+offset and a
type-casting the result. Rename to SPA_PTROFF which is more expressive (and
has the same number of characters so we don't need to re-indent).
Remove the hooks we installed on objects in the destroy/free event
or before calling _destroy. This is not really needed but it is
a nice thing to do because it calls the hook removed callbacks.
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.
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.
