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/IconView.h

@@ -70,7 +70,7 @@ private:
         Gfx::IntRect icon_rect;
         int icon_offset_y;
         int text_offset_y;
-        String text;
+        DeprecatedString text;
         Vector<StringView> wrapped_text_lines;
         ModelIndex index;
         bool valid { false };