Now that all the other junk is being dropped from the file, script info
lines are the only thing left which was a concrete AssEntry, and
AssEntry wasn't actually a very good way to store them.
Dealing with multiple instances of each section makes the code
significantly more complicated, and in most cases Aegisub doesn't
actually bother to handle it correctly.
Previously, Aegisub wrote its "Script generated by" lines without a
semicolon to mark them as comments. As a result, it prepended these
lines every time a script was loaded and then saved again. Fix.
Also fix a bug that caused the ASS parser to treat comments before
the [Script Info] section as non-blank lines, therefore inserting a
duplicate section header whenever such comments were present.
