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.
Previously merged in !208 (merged), but not actually appeared in the repository. Might have gotten lost when the gitlab database migration was done.