Enter submenus when activating their parent item

This gives consistent behavior with e.g. Qt, Mozilla's suites and
LibreOffice (with non-truly native backends like "gen" and "gtk",
but unlike "gtk2" and "gtk3" ones that probably use true GTK menus).

This behavior is expected by at least some accessibility users, and
it seems good to behave like other common applications and toolkits
in this area.  There should be no issue in doing so either for current
users, as it only enters the submenu instead of not doing anything.

https://bugzilla.gnome.org/show_bug.cgi?id=778811

gtk/gtkmenushell.c

@@ -1434,7 +1434,7 @@ gtk_real_menu_shell_activate_current (GtkMenuShell *menu_shell,
                                     priv->active_menu_item,
                                     force_hide);
     else
-      _gtk_menu_item_popup_submenu (priv->active_menu_item, FALSE);
+      gtk_menu_shell_select_submenu_first (menu_shell);
   }
 }