Add bindings for pw_properties

I created some basic bindings for pw_properties so that I can work on bindings for pw_stream.

There are still some TODOs for the more advanced function left, but I didn't care about implementing those yet.

It's a bit unfortunate that the only method for creating a Properties struct is a macro right now, but apparently it works even in forbid(unsafe_code) crates, and I don't have a better solution (at least without copying the Strings around multiple times) since we can't call a variadic C function with a dynamic number of elements.