A check-button menu entry behaves much like a check-button widget.
When it is invoked it toggles back and forth between the selected
and deselected states. When the entry is selected, a particular
value is stored in a particular global variable (as determined by
the :onvalue and :variable options for the entry); when
the entry is deselected another value (determined by the
:offvalue option) is stored in the global variable.
A selector box is displayed to the left of the label in a check-button
entry. If the entry is selected then the box's center is displayed
in the color given by the selector option for the menu;
otherwise the box's center is displayed in the background color for
the menu. If a :command option is specified for a check-button
entry, then its value is evaluated as a Tcl command each time the entry
is invoked; this happens after toggling the entry's