Add ui-multiselect-open class to multiselect component

[rsikdar]

I'm submitting a ... (check one with "x")

[ ] bug report => Search github for a similar issue or PR before submitting
[ x ] feature request => Please check if request is not on the roadmap already https://github.com/primefaces/primeng/wiki/Roadmap
[ ] support request => Please do not submit support request here, instead see http://forum.primefaces.org/viewforum.php?f=35

Hello, I would like to propose adding a ui-multiselect-open class to the multiselect component when the overlay is visible.

Expected behavior
The container div has a class "ui-multiselect-open" when the overlay is visible

What is the motivation / use case for changing the behavior?

The dropdown component already has a ui-dropdown-open class that I was using to style the dropdown differently when the overlay is visible, but I am unable to use similar styling with the multiselect component. Once the user clicks inside the overlay, the label container loses the ui-state-focus class, and there is no way through css to know the overlay is visible.

Thanks!

[rsikdar]

Created pull request for the feature - #5681