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.
Fixes clippy warnings, prevents confusing errors and is more consistent.
The Display trait provides a to_string() method by itself and FromStr
provides from_str().
