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/GStackWidget.h

@@ -0,0 +1,21 @@
+#pragma once
+
+#include <LibGUI/GWidget.h>
+
+class GStackWidget : public GWidget {
+public:
+    explicit GStackWidget(GWidget* parent);
+    virtual ~GStackWidget() override;
+
+    GWidget* active_widget() const { return m_active_widget; }
+    void set_active_widget(GWidget*);
+
+    virtual const char* class_name() const override { return "GStackWidget"; }
+
+protected:
+    virtual void child_event(CChildEvent&) override;
+    virtual void resize_event(GResizeEvent&) override;
+
+private:
+    GWidget* m_active_widget { nullptr };
+};