LibGUI: Add a new GComboBox widget.

This widget combines a GTextEditor, a GButton, a GWindow and a GListView
to implement a nice drop-down list.

It's currently using the GWindowType::Tooltip type because that's the most
appropriately behaving window type available at the moment. This should
definitely be fixed though.

LibGUI/GComboBox.h

@@ -0,0 +1,35 @@
+#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 open();
+    void close();
+
+    GModel* model() { return m_list_view->model(); }
+    const GModel* model() const { return m_list_view->model(); }
+    void set_model(NonnullRefPtr<GModel>);
+
+    Function<void(const String&)> on_change;
+
+    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 };
+};