Do proper unicode case-folding for case-insensitive searching rather
than converting only ascii characters to lowercase. The Turkish 'i' is
still not handled correctly (since it's the only place where
case-folding is locale-dependent), but that's probably not worth caring
about as long as we don't have a Turkish UI translation.
This affects both the find/replace dialog and the select lines dialog.
Closes#1342.
47c36c9033