Unify and refactor caller and callee GObject argument marshalers.
Combine code from the large switch statement used to marshal
arguments to and from vfuncs/closures with the marshalers used
for direct calls to gi functions. This fixes a reference leak
when marshalling GObjects to Python with transfer=full due to
the diverging code paths.
Replace ability in gobject_new_full to optionally sink objects
with ability to optionaly "steal" objects. This fits the premise
that binding layers should always sink objects initially. The
steal argument is then used for marshalling arguments which are
transfer=full.
Add hacks and comments to work around GTK+ bugs 693393 and 693400.

https://bugzilla.gnome.org/show_bug.cgi?id=687522