* \section nestedwidgetkinds Hierarchical widget kinds

 * <ul>

 * <li> #NWidgetLeaf for widgets visible for the user, or

 * <li> #NWidgetSpacer for creating (flexible) empty space between widgets.

 * </ul>

 * The purpose of a leaf widget is to provide interaction with the user by displaying settings, and/or allowing changing the settings.

 * <ul>

 * <li> #NWidgetHorizontal for organizing child widgets in a (horizontal) row. The row switches order depending on the language setting (thus supporting

 *      right-to-left languages),

 * <li> #NWidgetHorizontalLTR for organizing child widgets in a (horizontal) row, always in the same order. All childs below this container will also

 *      never swap order.

 * <li> #NWidgetVertical for organizing child widgets underneath each other.

 * <li> #NWidgetBackground for adding a background behind its child widget.

 * <li> #NWidgetStacked for stacking child widgets on top of each other.

 * </ul>

 * The purpose of a container widget is to structure its leafs and sub-containers to allow proper resizing.

 *

 * \section nestedwidgetscomputations Hierarchical widget computations

 * The first 'computation' is the creation of the nested widgets tree by calling the constructors of the widgets listed above and calling \c Add() for every child,

 * or by means of specifying the tree as a collection of nested widgets parts and instantiating the tree from the array.

 *

 * After the creation step,

 * - The leafs have their own minimal size (\e min_x, \e min_y), filling (\e fill_x, \e fill_y), and resize steps (\e resize_x, \e resize_y).

 * - Containers only know what their children are, \e fill_x, \e fill_y, \e resize_x, and \e resize_y are not initialized.

 *

 * Computations in the nested widgets take place as follows:

 * <ol>

 * <li> A bottom-up sweep by recursively calling NWidgetBase::SetupSmallestSize() to initialize the smallest size (\e smallest_x, \e smallest_y) and

 *      to propagate filling and resize steps upwards to the root of the tree.

 * <li> A top-down sweep by recursively calling NWidgetBase::AssignSizePosition() to make the smallest sizes consistent over the entire tree, and to assign

 *      the top-left (\e pos_x, \e pos_y) position of each widget in the tree. This step uses \e fill_x and \e fill_y at each node in the tree to decide how to

 *      fill each widget towards consistent sizes.

 *      For generating a widget array, resize step sizes are made consistent.

 * </ol>

 * @param top_moving  Top edge of the widget may move due to resizing.