add_check name onval offval variable command
A check button command is identical to a normal command created by
add_command except that the menu item also has a box showing the
current toggled state. Unlike radio buttons each check button operates
independently of every other check button.
The variable button specifies the global Tcl variable to hold the state for this check button. It will contain 1 for enabled and 0 for disabled. Whenever the item is selected the variable will be toggled and the command executed.