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.cpp

@@ -0,0 +1,92 @@
+#include <LibGUI/GButton.h>
+#include <LibGUI/GComboBox.h>
+#include <LibGUI/GListView.h>
+#include <LibGUI/GScrollBar.h>
+#include <LibGUI/GTextEditor.h>
+#include <LibGUI/GWindow.h>
+
+GComboBox::GComboBox(GWidget* parent)
+    : GWidget(parent)
+{
+    m_editor = new GTextEditor(GTextEditor::Type::SingleLine, this);
+    m_editor->on_change = [this] {
+        if (on_change)
+            on_change(m_editor->text());
+    };
+    m_open_button = new GButton(this);
+    m_open_button->set_focusable(false);
+    m_open_button->set_text("\xf7");
+    m_open_button->on_click = [this](auto&) {
+        if (m_list_window->is_visible())
+            close();
+        else
+            open();
+    };
+
+    m_list_window = new GWindow(this);
+    // FIXME: This is obviously not a tooltip window, but it's the closest thing to what we want atm.
+    m_list_window->set_window_type(GWindowType::Tooltip);
+    m_list_window->set_should_destroy_on_close(false);
+
+    m_list_view = new GListView(nullptr);
+    m_list_view->horizontal_scrollbar().set_visible(false);
+    m_list_window->set_main_widget(m_list_view);
+
+    m_list_view->on_selection = [this](auto& index) {
+        ASSERT(model());
+        auto new_value = model()->data(index).to_string();
+        m_editor->set_text(new_value);
+        m_editor->select_all();
+        m_list_window->hide();
+    };
+}
+
+GComboBox::~GComboBox()
+{
+}
+
+void GComboBox::resize_event(GResizeEvent& event)
+{
+    int frame_thickness = m_editor->frame_thickness();
+    int button_height = event.size().height() - frame_thickness * 2;
+    int button_width = 15;
+    m_open_button->set_relative_rect(width() - button_width - frame_thickness, frame_thickness, button_width, button_height);
+    m_editor->set_relative_rect(0, 0, width(), height());
+}
+
+void GComboBox::set_model(NonnullRefPtr<GModel> model)
+{
+    m_list_view->set_model(move(model));
+}
+
+void GComboBox::open()
+{
+    if (!model())
+        return;
+
+    auto my_screen_rect = screen_relative_rect();
+
+    int longest_item_width = 0;
+    for (int i = 0; i < model()->row_count(); ++i) {
+        auto index = model()->index(i);
+        auto item_text = model()->data(index).to_string();
+        longest_item_width = max(longest_item_width, m_list_view->font().width(item_text));
+    }
+    Size size {
+        max(width(), longest_item_width + m_list_view->width_occupied_by_vertical_scrollbar()) + m_list_view->frame_thickness() * 2 + m_list_view->horizontal_padding(),
+        model()->row_count() * m_list_view->item_height() + m_list_view->frame_thickness() * 2
+    };
+
+    m_list_window->set_rect({ my_screen_rect.bottom_left(), size });
+    m_list_window->show();
+}
+
+void GComboBox::close()
+{
+    m_list_window->hide();
+}
+
+String GComboBox::text() const
+{
+    return m_editor->text();
+}

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 };
+};

LibGUI/Makefile

@@ -60,6 +60,7 @@ LIBGUI_OBJS = \
     GRadioButton.o \
     GAbstractButton.o \
     GListView.o \
+    GComboBox.o \
     GWindow.o
 
 OBJS = $(SHAREDGRAPHICS_OBJS) $(LIBGUI_OBJS)