* gtk/gtkcombobox.c (gtk_combo_box_size_request),
(gtk_combo_box_size_allocate): Rework, things behave better now. Still
needs some work on requesting enough size when taking the items in
the list into account.
* gtk/gtkfilesel.c: put gtkoptionmenu.h include inside "enable
deprecated" guards.
* gtk/gtkinputdialog.c: ditto.
* gtk/gtkitemfactory.c: ditto.
* gtk/gtkoptionmenu.c: ditto.
* demos/gtk-demo/menus.c: remove the option menu.
* demos/gtk-demo/sizegroup.c: replace the option menus with
GtkComboBox.
* demos/gtk-demo/textview.c: ditto.
(A general GtkComboBox demo will be added later on).