A GtkToggleButton is a button that will retain
its state following activation. This can be either the active state,
GtkStateType GTK_STATE_ACTIVE,
or the normal state, GTK_STATE_NORMAL. The active
state has the appearance of a button that has been pressed down and not
released.
The "toggled" signal is
emitted as part of the process when the button is activated. Activation
also triggers a change from the current state to its opposite value.
GtkToggleButton also provides some base elements
for GtkCheckButton and
GtkRadioButton widgets.
