adjusted return behavior, corrected when there is a selection

previously, a return key only inserted a new line when at the end of a
non-blank line (use control+return key to always insert a new line);
adjusted behavior to also insert a new line on a blank line

when there was a selection, a return key ignored the selection an
advanced to the next line; this was corrected to replace the selected
text with a new line

when there was a selection across two lines with characters selected on
the second line, a control+return key correctly replaced the selection
with a new line, however, the edit box was not redrawn properly (the
selection remained on the screen, though text was not actually selected
and the text was removed from the document); this was corrected by
creating a new key press event with a no modified return key