Avoid unintentionally converting between float and double multiple times
by just using double everywhere. Also, remove the unused `int` versions
of their constructors.
This will make it easier to support both string types at the same time
while we convert code, and tracking down remaining uses.
One big exception is Value::to_string() in LibJS, where the name is
dictated by the ToString AO.
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 :^)
Which is to say, a T where `is_calculated()` is false.
As is becoming a repeating theme with CSS types, we have two states for
a FooPercentage that is a `calc()` expression: Either the FooPercentage
holds the CalculatedStyleValue directly, or it holds a Foo which itself
holds the CalculatedStyleValue. The first case was already handled to
return Foo, and with this patch, the second is too. :^)
