When the `FilteringOptions::SortByScore` flag is set, filtered indices
are sorted by match score in descending order, meaning the most
relevant results should appear first.
The default behavior of FilteringProxyModel is unchanged.
We have a new, improved string type coming up in AK (OOM aware, no null
state), and while it's going to use UTF-8, the name UTF8String is a
mouthful - so let's free up the String name by renaming the existing
class.
Making the old one have an annoying name will hopefully also help with
quick adoption :^)
