GtkToolShell: Move vfunc default results to default implementations.

GtkToolShell: Move vfunc default results to default implementations.

Otherwise the default values are only used if no vfunc is set,
making that default unavailable to vfunc implementations as a fallback,
and unavailable to language bindings (such as gtkmm) that always
have a vfunc implementation, even if that often only tries to call
the base vfunc implemntation.

This was making gtkmm's ToolPalette's (and probably ToolBar's)
items look like buttons instead of being flat.

https://bugzilla.gnome.org/show_bug.cgi?id=656223