Refactor|libappfw: More robust child widget virtualization for menus

Since filtering is no longer a concern for ChildWidgetOrganizer, it
can now use simpler and more robust virtualization logic.

This corrects any issues when scrolling to arbitrary locations in
the item set. However, handling varying item heights still requires
a bit more finesse.

doomsday/sdk/libappfw/include/de/framework/childwidgetorganizer.h

@@ -76,28 +76,6 @@ class LIBAPPFW_PUBLIC ChildWidgetOrganizer
         virtual void updateItemWidget(GuiWidget &widget, ui::Item const &item) = 0;
     };
 
-    /**
-     * Filters out data items.
-     */
-    class IFilter
-    {
-    public:
-        virtual ~IFilter() {}
-
-        /**
-         * Determines whether an item should be accepted or ignored by the organizer.
-         *
-         * @param organizer  Which organizer is asking the question.
-         * @param data       Data context.
-         * @param item       Item to check.
-         *
-         * @return @c true to accept item, @c false to ignore it.
-         */
-        virtual bool isItemAccepted(ChildWidgetOrganizer const &organizer,
-                                    ui::Data const &data,
-                                    ui::Item const &item) const = 0;
-    };
-
 public:
     ChildWidgetOrganizer(GuiWidget &container);
 
@@ -112,15 +90,6 @@ class LIBAPPFW_PUBLIC ChildWidgetOrganizer
 
     IWidgetFactory &widgetFactory() const;
 
-    /**
-     * Sets the object that decides whether items are accepted or ignored.
-     *
-     * @param filter  Filtering object.
-     */
-    void setFilter(IFilter const &filter);
-
-    void unsetFilter();
-
     /**
      * Sets the data context of the organizer. If there was a previous context,
      * all widgets created for it are deleted from the container. The widgets
@@ -140,17 +109,6 @@ class LIBAPPFW_PUBLIC ChildWidgetOrganizer
 
     ui::Item const *findItemForWidget(GuiWidget const &widget) const;
 
-    /**
-     * Filters all items according to the defined IFilter. Widgets are
-     * created and removed as needed according to the filter.
-     */
-    void refilter();
-
-    /**
-     * Number of items currently being organized. This is affected by the filtering.
-     */
-    dsize itemCount() const;
-
 //- Child Widget Virtualization ---------------------------------------------------------
 
     /**
@@ -166,6 +124,8 @@ class LIBAPPFW_PUBLIC ChildWidgetOrganizer
      */
     void setVirtualizationEnabled(bool enabled);
 
+    void setVirtualTopEdge(Rule const &topEdge);
+
     void setVisibleArea(Rule const &minimum, Rule const &maximum);
 
     bool virtualizationEnabled() const;