gdbus-codegen: Rework C property getters

gdbus-codegen: Rework C property getters

Rework property getters to use a vfunc so we can take the fast path
and avoid allocating memory for both the skeleton and the proxy
cases. This requires some special case because of how GVariant expects
you to free memory in some cases, see #657100. Add test cases for
this.

Document the _get_ functions as not being thread-safe and also
generate _dup_ C getters (which are thread-safe).

Mark all the generated _get_, _dup_ and _set_ as (skip) as non-C
languages should just use GObject properties and not the (socalled)
"C binding".

Signed-off-by: David Zeuthen <davidz@redhat.com>