ComboBox - a ComboBox widget for Tk4.0

DESCRIPTION
     This Tcl code implements a basic ComboBox widget for Tk4.0.
     The clever bit is  that you can access the widget's
     internals just like you would any other Tk4.0 widget (well,
     almost :-). This is accomplished by creating a new proc with
     the same name as the widget. This proc lets the programmer
     do things like "<widget-name> configure" and <widget-name>
     cget" and so on.
