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 :^)
With this change you can now set the theme and background color at the
same time in the Display Settings. Before if both were changed
before hitting 'apply' the theme background color would overwrite
the custom background.
