p-calendar add clear button

[tigrenok00]

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

Please add a link/button inside the control for the user to clear/reset the value after selection.
I saw that the issue came up in several places on the forum, but no solution...
Thanks!

[koesper]

The wish for a 'clear' button was also expressed in #506

[jayordway]

When is this clear button coming, 506 was closed. I still see no clear button.

[cagataycivici]

Duplicate of #506