1. Polish variables in EditorWindow extension loading and Tkinter variable code.

1. Polish variables in EditorWindow extension loading and Tkinter variable code.
2. Add exception handling to EditorWindow Tkinter variable setvar() and getvar() fcns.
3. EditorWindow: remove some unneeded comments.
4. Add a separator to the Options menu
5. extend.txt: describe how to create a menu entry which has no keybinding.

M Bindings.py
M EditorWindow.py
M extend.txt