[plt-scheme] slightly less pre-alpha debugger

The debugger has taken another baby step towards usefulness today with 
the addition of a slightly less appalling method for setting 
breakpoints.

In particular, there is now a 'Debugger' menu containing three menu 
items: Set Breakpoint, List Breakpoints, and Clear All Breakpoints.
Choosing the 'Set Breakpoint' item sets a breakpoint at the current 
insertion point, or at the beginning of the current selection.

The debugger simply remembers a list of source file offsets; inserting 
characters before, after, or between existing breakpoints will not 
change these offsets.  If a listed breakpoint does not correspond to 
the beginning of a source expression, that breakpoint will be ignored.

Setting or clearing breakpoints while the program is running has no 
effect.

To run the debugger, you must click on the 'Debug' button.

To see a list of commands, type (help) in the debugger's REPL (yes, the 
one that says MrEd: Quit now etc. etc.).

The debugger inserts an implicit breakpoint after each top-level 
expression, so that you can see the result of each expression's 
evaluation.

Comments & Additions always welcome.

john clements