See extended description

1. Removed plispgui from makefile
2. Added new function list() to avoid confusing nested cons()
3. Corrected placement of call to create_transcript_window() in main()
4. Added a bunch of UI functionality
5. Added OBJECT_PTRs for symbols LET, COND, DOTIMES and DOLIST
6. Added convenience functions first(), second()...
7. print_to_string() and related functions changed to handle indents
better
8. Unit test #311 updated to reflect the fact that symbols are now being
printed without package qualifier