[Accessibility] SHOW_MENU action should be handled in Node

Currently the code in Control handles the accessibility SHOW_MENU action, enabling controls to display their context menus when a user invokes a special accessibility shortcut (e.g. Ctrl+NumPad5 on Mac).
However, FX API allows any Node to add a context menu event handler and process it whenever the event occurs. Thus, any instance of the Node class may display the context menu. To make all nodes accessible, the SHOW_MENU handler should be moved from Control to Node. E.g., the code in Node could look like:

    public void accExecuteAction(Action action, Object... parameters) {
        if (Action.SHOW_MENU.equals(action)) {
            Bounds b = getBoundsInLocal();
            Point2D pt = localToScreen(b.getMaxX(), b.getMaxY());

            ContextMenuEvent context = new ContextMenuEvent(ContextMenuEvent.CONTEXT_MENU_REQUESTED,
                    b.getMaxX(), b.getMaxY(), pt.getX(), pt.getY(), false, new PickResult(this, b.getMaxX(), b.getMaxY()));
            Event.fireEvent(this, context);
        }
    }

(perhaps with some adjustments to the way the coordinates and the PickResult are constructed...)