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/GComboBox.h

@@ -0,0 +1,42 @@
+#pragma once
+
+#include <LibGUI/GListView.h>
+#include <LibGUI/GWidget.h>
+
+class GButton;
+class GTextEditor;
+
+class GComboBox : public GWidget {
+public:
+    explicit GComboBox(GWidget* parent = nullptr);
+    virtual ~GComboBox() override;
+
+    String text() const;
+    void set_text(const String&);
+
+    void open();
+    void close();
+    void select_all();
+
+    GModel* model() { return m_list_view->model(); }
+    const GModel* model() const { return m_list_view->model(); }
+    void set_model(NonnullRefPtr<GModel>);
+
+    bool only_allow_values_from_model() const { return m_only_allow_values_from_model; }
+    void set_only_allow_values_from_model(bool);
+
+    Function<void(const String&)> on_change;
+    Function<void()> on_return_pressed;
+
+    virtual const char* class_name() const override { return "GComboBox"; }
+
+protected:
+    virtual void resize_event(GResizeEvent&) override;
+
+private:
+    GTextEditor* m_editor { nullptr };
+    GButton* m_open_button { nullptr };
+    GWindow* m_list_window { nullptr };
+    GListView* m_list_view { nullptr };
+    bool m_only_allow_values_from_model { false };
+};