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/CheckBox.h

@@ -30,7 +30,7 @@ public:
     void set_checkbox_position(CheckBoxPosition value) { m_checkbox_position = value; }
 
 protected:
-    explicit CheckBox(String = {});
+    explicit CheckBox(DeprecatedString = {});
 
 private:
     void size_to_fit();