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 :^)
Previously, if you opened MouseSettings, set the cursor theme to Dark,
and then click "Defaults", the cursors list would not update.
ComboBox::set_text() does not call the on_change callback, so we have
to run the steps manually.
I've also made `m_theme_name` into a local variable, since it can be.