Tweak open_line_below to operate "properly".

open_line_below was previously inserting a newline before
the terminating newline of the current line. We now properly
insert the newline AFTER the terminating newline of the
current line. This is the "correct" behavior, and lets us look
at the character where the insertion is occurring to decide
what fold to use as with other situations.
In theory, a fully correct version of deciding folds on text insertion.

We now use the fold that was at the original insertion point if
we're inserting regular text, but if the text being inserted
ends in a newline and it's immediately after another newline,
then we use the fold *before* the insertion.