changed behaviour of left and right editors cmds to allow moving up the tree
Improved evalNode exception handling: lookUp and divZero
More evalNode exception handling: let syntax and call of non-procedure
fixed let to accept assignment of expression to vars, probably broke syntax exception handling
restricted editors to only draw in their window
fixed exception handling for let and lambda syntax
Move startup code into the WindowManager class
Moved some of the clean up code into WindowManager, delete Dlist
Added basic cells implementation
added right side operand overloading