Move editor customizations from "make.scm" to "edextra.scm". Extend

Move editor customizations from "make.scm" to "edextra.scm".  Extend
filename customizations so that they do the usual thing when the files
being operated on are not part of the student environment.  Add
remaining backup and auto-save filename customizations.  Add
Scheme's file types to list of ignored types.  Disable key bindings
that exit the editor.