These classes are used as-is in all chromes. Move them to LibWebView so
that non-Serenity chromes don't have to awkwardly reach into its headers
and sources.
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.
In 7c5e30daaa, the focus was "only" on
Userland/Libraries/, whereas this commit cleans up the remaining
headers in the repo, and any new badly-formatted include.
