Also check out the tree.gui presentation in the demo/examples directory. It lets you browse this hierarchy interactively. You need to specify a mime-type in your browser to make the presentation appear by clicking on this link.
Widget
ContainerWidget
GroupWidget
Presentation
PageWidget
PopupWidget
Tip
PopupMenu
DropDownMenu
GroupBoxWidget
FolderWidget
WindowWidget
ScrollingContainerWidget
TreeWidget
TableWidget
SparseTableWidget
ListWidget
ButtonWidget
CommandButtonWidget
MenuButtonWidget
URLButtonWidget
CheckboxWidget
OptionWidget
TextWidget
TextBoxWidget
StaticTextWidget
DropDownTextBoxWidget
RichTextBoxWidget
ChoiceWidget
ListBoxWidget
ComboBoxWidget
DropDownListBoxWidget
DropDownComboBoxWidget
ValueWidget
ScrollbarWidget
ProgressIndicatorWidget
SpinBoxWidget
SliderWidget
AnimatedWidget
AnimatedTextWidget
AnimatedImageWidget
ShapeWidget
LineWidget
RectangleWidget
OvalWidget
PolygonWidget
ResourceWidget
ImageWidget
AudioWidget
PresentationWidget
AWTWidget
ComponentWidget
AppletWidget
PopupMenuItemWidget
ListItemWidget
TreeNodeWidget
ListBoxItemWidget