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 :^)
This is a helpful option to prevent unwanted side effects, distinguish
between user and programmatic input, etc. Sliders and SpinBoxes were
implementing it idiosyncratically, so let's generalize the API and
give Buttons and TextEditors the same ability.
ValueSlider is a more generalized version of OpacitySlider when we need
a slider with values displayed. It will always show the current value
with a user defined suffix.
