View_DropButton: fixes

Fix DropButton. Some issues showed up when you upgraded this to use View_Popover instead of Flyout. Now it works nicely.

Example usage:
$this->add('View_DropButton', null, 'User_Menu')
  ->setHTML('<strong>Welcome</strong>, '.$this->auth->model['name'])
  ->useMenu()
    ->addMenuItem('#',      'Messages')
    ->addMenuItem('#',      'Settings')
    ->addMenuItem('logout', 'Sign Out')
  ;