pipewire: Make PropertiesRef struct into a transparent wrapper

Before this commit, PropertiesRef was not a transparent wrapper like the other ref structs in the crate are, so you couldn't use it in places were you just had a pointer to a nonowned pw_sys::pw_properties.

Deref/DerefMut impls are added to the Properties struct so that it can be converted automatically as needed.

The dict traits unfortunately still need to be implemented on both of these despite the Deref impl, but I'll change dicts to use the ref-struct pattern in the future and the traits will no longer be needed, then all functionality can live on PropertiesRef only.