modified edit box not to display a line number on a new line

for a new line not yet inserted into the program, a blank line number is
displayed (currently with a '+') until the line is entered (cursor moved
away from the line); this allows the line numbers in the edit box to
match the line numbers in the program view

this change also required the line number area of the edit box to be
redrawn (updated) when the line is finally inserted so that the line
numbers on the line below the new line can be updated