Libraries: Create top level directory for libraries.

Things were getting a little crowded in the project root, so this patch
moves the Lib*/ directories into Libraries/.

Libraries/LibGUI/GRadioButton.h

@@ -0,0 +1,31 @@
+#pragma once
+
+#include <LibGUI/GAbstractButton.h>
+
+class GRadioButton : public GAbstractButton {
+public:
+    GRadioButton(const StringView& text, GWidget* parent);
+    virtual ~GRadioButton() override;
+
+    virtual const char* class_name() const override { return "GRadioButton"; }
+
+    virtual void click() override;
+
+protected:
+    virtual void paint_event(GPaintEvent&) override;
+
+private:
+    virtual bool is_radio_button() const final { return true; }
+
+    template<typename Callback>
+    void for_each_in_group(Callback);
+    static Size circle_size();
+};
+
+template<>
+inline bool is<GRadioButton>(const CObject& object)
+{
+    if (!is<GWidget>(object))
+        return false;
+    return to<GWidget>(object).is_radio_button();
+}