The patch I will attach adds a new GObject virtual method that intends to
replace GObjectClass->constructor, which currently has two problems:

  1. It is not possible to chain to parent constructor, since it is assumed that
the parent type is kept static in a function or as a global variable; thus it is
not possible in run-time to distinguish between types all sharing the same
constructor C function;

  2. It is not possible to pass arbitrary data into the constructor; In pygtk
bug #161177 we need to pass to the constructor information on whether a Python
wrapper for a GObject has already been created or not;  And we need this in the
constructor because you're not allowed to change construct-only properties
outside of it, so this is also related to bug #123891.

  Thus, a new constructor_full virtual is proposed, to be used together with the
new API g_object_newv_with_data.