sanitize show var/code handling; switch to enum for future changes

gui.html

@@ -149,7 +149,7 @@ dimension is expected, or the removal of a variable that's still used
 somewhere.
 <P>
 If the location of the error is not obvious, the list of objects can be
-shown by selecting "Swap var&amp;code" from the View menu. The object
+shown by selecting "Show code" from the View menu. The object
 in which the error occurred is shown in red. If the error occurred in a
 loop variable, the variable name is shown in red.