Else, outline items that have newlines in them end up with a weird
vertical offset.
(This does affect the outline item's tooltip, which shows the whole
title. But not having a newline there seems alright, arguably
preferable.)
The title of an OutlineItem is already in UTF-8.
This is currently done in LibPDF's Parser::parse_string(). I think
that's not quite the right place (it shouldn't be done for all strings)
and not done quite right (text strings should convert from
PDFDocEncoding to UTF-8 unless prefixed by an UTF-8 BOM), but even if
that changes, I think we'll keep OutlineItem.title in UTF-8.
