iconview: Redo size requests

Instead of just returning the last allocated numbers, we now compute the
proper sizes from scratch. This is a bit less trivial, but it results in
proper height-for-width handling.

gtk/gtkiconviewprivate.h

@@ -50,8 +50,6 @@ struct _GtkIconViewPrivate
 
   GtkSelectionMode selection_mode;
 
-  guint layout_idle_id;
-
   GdkWindow *bin_window;
 
   GList *children;