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 :^)
Instead of asking Gfx::FontDatabase for the "default font" and the
"default fixed-width font", we now proxy those requests out via
the Platform::FontPlugin. This will allow Ladybird to use other default
fonts as fallback.
Instead of hard-coding the names of system fonts to use for the CSS
generic fonts (like "sans-serif", "monospace", etc.) we now call out
to a Platform::FontPlugin and ask for the generic names.