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.