GTK 3 prep: stop using *nearly* all GTK deprecated functions.

Building with -DGTK_DISABLE_DEPRECATED, we now suffer only one compile
failure, for the use of gtk_quit_add() in idle_toplevel_callback_func.
That function is apparently removed with no replacement in GTK 3, so
I'll need to find a completely different approach to getting toplevel
callbacks to run only in the outermost instance of gtk_main().

Also, this change doesn't do anything about the use of *GDK*
deprecated functions, because those include the entire family of
old-style drawing functions - i.e. the only way to build cleanly with
-DGDK_DISABLE_DEPRECATED will be to switch to Cairo drawing.

unix/gtkfont.c

@@ -2190,7 +2190,8 @@ static void unifontsel_select_font(unifontsel_internal *fs,
      * Grey out the font size edit box if we're not using a
      * scalable font.
      */
-    gtk_entry_set_editable(GTK_ENTRY(fs->size_entry), fs->selected->size == 0);
+    gtk_editable_set_editable(GTK_EDITABLE(fs->size_entry),
+                              fs->selected->size == 0);
     gtk_widget_set_sensitive(fs->size_entry, fs->selected->size == 0);
 
     unifontsel_draw_preview_text(fs);