LibGUI: The UI calls it 'Icon view', fix the name in the code

Author: https://github.com/BenWiederhake
Commit: ef433cb367
Pull-request: https://github.com/SerenityOS/serenity/pull/2036
Reviewed-by: https://github.com/awesomekling
Reviewed-by: https://github.com/zlotny

Libraries/LibGUI/Forward.h

@@ -40,7 +40,7 @@ class FileSystemModel;
 class Frame;
 class GroupBox;
 class HorizontalBoxLayout;
-class ItemView;
+class IconView;
 class JsonArrayModel;
 class KeyEvent;
 class Label;