app, menus: add an application menu for systems which wants this.

In particular on macOS, we want to show some default common actions (see
comments in !837) but gtk_application_prefers_app_menu() docs says that it will
always return FALSE on this OS. So we ignore this call on macOS.

app/display/gimpimagewindow.c

@@ -351,6 +351,7 @@ gimp_image_window_constructed (GObject *object)
   GimpGuiConfig          *config;
   GimpMenuModel          *model;
   gboolean                use_gtk_menubar = TRUE;
+  gboolean                use_app_menu    = TRUE;
 
   G_OBJECT_CLASS (parent_class)->constructed (object);
 
@@ -425,6 +426,25 @@ gimp_image_window_constructed (GObject *object)
                         window);
     }
 
+#ifndef GDK_WINDOWING_QUARTZ
+  /* Docs says that macOS always returns FALSE but we actually want to create
+   * our custom macOS menu.
+   */
+  use_app_menu = gtk_application_prefers_app_menu (GTK_APPLICATION (private->gimp->app));
+#endif /* !GDK_WINDOWING_QUARTZ */
+
+  if (use_app_menu)
+    {
+      GimpUIManager *app_menu_manager;
+      GimpMenuModel *app_menu_model;
+
+      app_menu_manager = gimp_menu_factory_get_manager (menus_get_global_menu_factory (private->gimp),
+                                                        "<AppMenu>", private->gimp);
+      app_menu_model   = gimp_ui_manager_get_model (app_menu_manager, "/app-menu");
+      gtk_application_set_app_menu (GTK_APPLICATION (private->gimp->app),
+                                    G_MENU_MODEL (app_menu_model));
+    }
+
   /* Create the hbox that contains docks and images */
   private->hbox = gtk_box_new (GTK_ORIENTATION_HORIZONTAL, 0);
   gtk_box_pack_start (GTK_BOX (private->main_vbox), private->hbox,

app/menus/menus.c

@@ -98,6 +98,14 @@ menus_init (Gimp *gimp)
                                       "quick-mask-menu", NULL,
                                       NULL);
 
+  gimp_menu_factory_manager_register (global_menu_factory, "<AppMenu>",
+                                      "file",
+                                      "dialogs",
+                                      NULL,
+                                      "/app-menu",
+                                      "app-menu", NULL,
+                                      NULL);
+
   gimp_menu_factory_manager_register (global_menu_factory, "<Toolbox>",
                                       "file",
                                       "context",

menus/Makefile.am

@@ -10,6 +10,7 @@ menudata_built_files = $(menudata_in_files:.ui.in.in=.ui)
 
 menudata_DATA = \
 	$(menudata_built_files)		\
+	app-menu.ui			\
 	brush-editor-menu.ui		\
 	brushes-menu.ui			\
 	buffers-menu.ui			\

menus/app-menu.ui

@@ -0,0 +1,14 @@
+<?xml version="1.0" encoding="utf-8"?>
+
+<interface>
+  <menu id="/app-menu">
+    <item><attribute name="action">app.dialogs-about</attribute></item>
+    <item><attribute name="action">app.dialogs-welcome</attribute></item>
+    <section>
+      <item><attribute name="action">app.dialogs-preferences</attribute></item>
+      <item><attribute name="action">app.dialogs-input-devices</attribute></item>
+      <item><attribute name="action">app.dialogs-keyboard-shortcuts</attribute></item>
+    </section>
+    <item><attribute name="action">app.file-quit</attribute></item>
+  </menu>
+</interface>

menus/meson.build

@@ -1,6 +1,7 @@
 menus_dir = prefix / gimpdatadir / 'menus'
 
 ui_menus_files = files(
+  'app-menu.ui',
   'brush-editor-menu.ui',
   'brushes-menu.ui',
   'buffers-menu.ui',