This procedure creates an option menubutton whose name is
pathName, plus an associated menu. Together they allow the
user to select one of the values given by the value
arguments. The current value will be stored in the global variable
whose name is given by varName and it will also be displayed
as the label in the option menubutton. The user can click on the
menubutton to display a menu containing all of the values
and thereby select a new value. Once a new value is selected, it
will be stored in the variable and appear in the option menubutton.
The current value can also be changed by setting the variable.
The return value from tk_optionMenu is the name of the
menu associated with pathName, so that the caller can change
its configuration options or manipulate it in other ways.