WidgetGroup[{content, ...}] specifies custom layout settings for how the content widgets are displayed.
WidgetSpace[] is a widget layout placeholder used when spacing is needed between widgets.
WidgetToSymbolicGUIKitXML[expr] converts a user interface expression to a GUIKitXML SymbolicXML expression.
Widget["ComboBox"] represents a combo box.
Widget["List"] represents a list.
Widget["Panel"] is an empty container widget.
Widget["PopupMenu"] represents a popup menu.
Widget["RadioButton"] represents a radio button.
Widget["TextArea"] represents a text area.
Widget["ToolBar"] represents a toolbar.