gdbus-codegen: Generate specific marshallers for each required type
As requested in #3028 (closed), generate marshallers for all the signals that are generated both for handling method calls and for emitting them.
Re-use the glib marhsallers when possible, otherwise define new ones that can be reused across all the signal/methods of the various interfaces defined in a c file.
Use direct gvalue access as we do in other private implementations.
Added tests.
/cc @chergert