Abstract base class for GtkMenuItem, GtkListItem, and GtkTreeItem.
GtkItem provides the toggle() method and "select", "deselect" and "toggle" signal prototypes for GtkMenuItem, GtkListItem, and GtkTreeItem.
Note that GtkItem is an abstract base class and cannot be constructed directly.
select() Emits "select" signal. deselect() Emits "deselect" signal. toggle() Emits "toggle" signal.
"select" Emitted when user selects item using mouse-click. "deselect" Emitted when user deselects item using mouse-click. "toggle" Emitted when user selects or deselects item using Enter key.