Restore return-raw-pointer logic

Some Gtk object constructor return value type is GtkWidget instead of
real type, for example, gtk_window_new returns GtkWidget instead of
GtkWindow.  This needs to be worked around via original
return-raw-pointer logic, which is removed by mistake.  This patch
restore it.
Merge pull request #37 from hying-caritas/restore-return-raw-pointer

Restore return-raw-pointer logic