Starts a GUI console implemented via a Tk text widget. This should
probably be called at most once per session. Also redefines the file
pager (as used by help()) to be the Tk pager.
Note
tkStartGUI() saves its evaluation environment as
.GUIenv. This means that the user interface elements can be
accessed in order to extend the interface. The three main objects are
named Term, Menu, and Toolbar, and the various
submenus and callback functions can be seen with
ls(envir = .GUIenv).