LibGUI: Make class_name() public so you can always call it.

I found myself having to cast to GWidget* all the time when writing some
generic debugging code that just wanted to dump widget info.

LibGUI/GFilePicker.h

@@ -5,6 +5,7 @@ public:
     GFilePicker();
     virtual ~GFilePicker() override;
 
-private:
     virtual const char* class_name() const override { return "GFilePicker"; }
+
+private:
 };