Feature request: Add emptyMessage for Dropdown with filter enabled

[jc-white]

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

Current behavior
When no items in a dropdown match the filter, a blank box shows.

Expected behavior
Would be nice to show a custom emptyMessage when no items match the typed filter.

What is the motivation / use case for changing the behavior?
Better UX for Dropdowns

[jc-white]

Thank you! Any chance we can get the same treatment for Multiselect?