check if column[i].button is valid before calling gtk_widget_intersect on

check if column[i].button is valid before calling gtk_widget_intersect on

* gtk/gtkclist.c: check if column[i].button is valid before calling
gtk_widget_intersect on it in gtk_clist_draw

* gtk/gtkdnd.c: removed unused variable empty_bits

* gtk/gtkrc.c: we should use gslist_* functions with GSLists...

* gtk/gtktypeutils.[ch]: makes more send for gtk_type_is_a to return
a boolean (thanks to Elrond for noticing this)

-Yosh