editbox: give edit box pointer to program unit instance

the edit box class constructor was modified to be given a pointer to the
program unit being assigned to the edit box (which is stored in a new
member variable), and to make the connection for line changes signal (to
program unit) and for the errors list changed signal (from program unit)

the main window constructor was modified to create the program model
first before creating the edit box, which is now given a pointer to the
program model