Add functions
pw_properties_steal(properties, key);
pw_properties_gift(properties, key, value);
which can be used to retrieve/insert key-value pairs from/into
a property list without creating unnecessary copies of the value.
Add tests as well.
Add
pw_properties_move(dst, dst_key, src, src_key, fallback_value)
which can move a key-value pair from one property list into
another, optionally using the fallback value as value if
`src_key` cannot be found in `src`.
Add tests as well.
This commit also adds `pw_properties_rename()`, which can be used
to change the key in a dictionary, but that is not made part of
the public API yet.
