Change gimp_dialog_factory_set_constructor() to
gimp_dialog_factory_set_put_in_dockables() order to narrow the
interface a bit. We can make both
gimp_dialog_factory_set_put_in_dockables() and the
GimpDialogConstructor typedef internal this way.

The main reason we do this is because we want to get rid of a
dependency on factory->p->new_dock_func. Eventually we want to
construct docks just like we construct other widgets in the factory,
so new_dock_func will be removed.

Also improve readability of code such as making it explicit that
gimp_dialog_factory_put_in_dockable_constructor() is just an extended
version of gimp_dialog_factory_default_constructor().