Suggestion: Add a reference to the monthpicker element for OnBeforeMenuOpen

Currently, the `OnBeforeMenuOpen` event only has the `event` argument. It would be more convenient for us if it has arguments like jQuery UI Datepicker's [`beforeShow`](https://api.jqueryui.com/datepicker/#option-beforeShow) do so that we can reference the monthpicker element (`.month-picker`).