added line numbers to the edit box

for now the line numbers start at 0 to match the actual line numbers
  (for debugging, this can easily be changed to 1 later)

a line number widget was added to the left of the edit box contents
where the line numbers are drawn to with one space to either side
of the line numbers (the width of the widget is only large enough
to hold the maximum line number of the document plus a space on
both sides of the line numbers)