AK+Everywhere: Rename String to DeprecatedString

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 :^)

Userland/Libraries/LibGUI/TextBox.cpp

@@ -67,7 +67,7 @@ void TextBox::add_current_text_to_history()
     m_saved_input = {};
 }
 
-void TextBox::add_input_to_history(String input)
+void TextBox::add_input_to_history(DeprecatedString input)
 {
     m_history.append(move(input));
     m_history_index++;