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