Rather than going through a ton of work to ensure that every AssEvent
has a sensible start time that happens to leave it in the right place
after sorting, simply sort only consecutive blocks of AssDialogues and
leave everything else untouched. Note that this results in different
behavior when there are multiple [Events] sections in a file -- rather
than moving lines between sections and keeping the number of lines per
section constant, each section keeps the lines it had.
As it's now easy to do, also add sorting by style name and end time.
Closes#614.
Originally committed to SVN as r4307.
2124a1dbd3