Daniel Boles authored Jan 12, 2021 and Kjell Ahlstedt committed Jan 12, 2021

Allow users to call Glib::manage(RefPtr<Binding>) in order to defeat our
pre-existing and still default behaviour that a user must keep a RefPtr
to the Binding in order to keep it alive, instead letting it revert to
the C-style behaviour of staying alive as long as either object does.

We use Glib::manage() to avoid the possible pitfall of a user calling
->set_manage() on the result of bind_property*(), which might be empty.

This patch has been modified by Kjell Ahlstedt.
This modified version stores the new instance data in a GQuark
instead of new member data, which would have broken ABI.

Fixes GNOME/glibmm#62