gtk3: use a more straightforward ownership for menu

Change-Id: Ibf2184f565225423f57597fe96164565b5a3194a
(cherry picked from commit 859c2874)

vcl/unx/gtk3/gdi/gtk3salnativewidgets-gtk.cxx

@@ -2009,23 +2009,23 @@ GtkSalGraphics::GtkSalGraphics( GtkSalFrame *pFrame, GtkWidget *pWindow )
 
     getStyleContext(&mpCheckButtonStyle, gtk_check_button_new());
 
+    /* Menu bar */
+    gMenuBarWidget = gtk_menu_bar_new();
+    gMenuItemMenuBarWidget = gtk_menu_item_new_with_label( "b" );
+    gtk_menu_shell_append(GTK_MENU_SHELL(gMenuBarWidget), gMenuItemMenuBarWidget);
+    getStyleContext(&mpMenuBarStyle, gMenuBarWidget);
+    mpMenuBarItemStyle = gtk_widget_get_style_context(gMenuItemMenuBarWidget);
+
     /* Menu */
     GtkWidget *menu = gtk_menu_new();
     mpMenuStyle = gtk_widget_get_style_context(menu);
-    g_object_ref_sink(menu);
+    gtk_menu_item_set_submenu(GTK_MENU_ITEM(gMenuItemMenuBarWidget), menu);
 
     /* Menu Items */
     gCheckMenuItemWidget = gtk_check_menu_item_new_with_label("M");
     gtk_menu_shell_append(GTK_MENU_SHELL(menu), gCheckMenuItemWidget);
     mpCheckMenuItemStyle = gtk_widget_get_style_context(gCheckMenuItemWidget);
 
-    /* Menu bar */
-    gMenuBarWidget = gtk_menu_bar_new();
-    gMenuItemMenuBarWidget = gtk_menu_item_new_with_label( "b" );
-    gtk_menu_shell_append(GTK_MENU_SHELL(gMenuBarWidget), gMenuItemMenuBarWidget);
-    getStyleContext(&mpMenuBarStyle, gMenuBarWidget);
-    mpMenuBarItemStyle = gtk_widget_get_style_context(gMenuItemMenuBarWidget);
-
     /* Spinbutton */
     gSpinBox = gtk_spin_button_new(NULL, 0, 0);
     getStyleContext(&mpSpinStyle, gSpinBox);