Add `Property::get_proxy() const` to get a `PropertyProxy_ReadOnly`
Glib::Property has a non-
get_proxy() that returns a
Glib::PropertyProxy for itself, but there's no
const overload, so in order to provide a
const proxy getter, we have to do it ourselves, and differently, by manually constructing a
Shouldn't we also provide a
Glib::Property::get_proxy() const -> Glib::PropertyProxy_ReadOnly to provide symmetry here?