Add `Property::get_proxy() const` to get a `PropertyProxy_ReadOnly`
Glib::Property
has a non-const
method 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 PropertProxy{this, "prop-name"}
.
Shouldn't we also provide a Glib::Property::get_proxy() const -> Glib::PropertyProxy_ReadOnly
to provide symmetry here?