Add various glib::Value trait impls for borrowed versions of types

For SDPMessageRef, StructureRef and CapsFeatureRef this involves the conversion from and to a glib::Value. Specifically this means that e.g. a StructureRef can be retrieved from a glib::Value via borrowing, i.e. without creating a copy of the structure first.

For all mini objects only retrieval from a glib::Value is implemented as the other direction would allow to get multiple immutable references from a mutable reference without borrowing, which is not allowed and would make it possible to observe a mini object changing while having an immutable reference to it.