Libdoc: New approach to implement search (#1872)

Technical changes: Earlier keywords were searched from the model and
their info re-rendered and then matches highlighted. Now highlight is
done first, keywords without highlighting marked as unmatched, and
styles used to format them.

UI changes: All shortcuts are shown always, unmatched are just
gray. Also all keyword docs are shown by default, unmatched as gray,
but there's an option to hide unmatched.
Libdoc: Styling search UI (#1872)
Libdoc: Make it impossible to disable last search include (#1872)
Libdoc: Column size tuning, hide search when printing
Libdoc search: Hide unmatched by default (#1872)