Add showClear property to MultiSelect

[SadiaWaleem]

I'm submitting a feature request in MultiSelect to clear the selected items.. (check one with "x")

[ ] bug report
[x ] feature request
[ ] support request => Please do not submit support request here, instead see https://forum.primefaces.org/viewforum.php?f=57

Current behavior
Currently, If multiple values are selected in MultiSelect component, the 'x' icon on dropdown is only closing the drop down not clearing the selected items in the list.

Expected behavior
Provide a feature that user can clear the selected items in MultiSelect like we used to do in DropDown by setting the showClear property to "true".

Minimal reproduction of the problem with instructions
In PrimeReact multiselect component, user can select the items from the list, the 'x' icon is using just to close the dropdown. Although in other components like in dropdown etc, 'x' is used to clear the chosen value along with closing the dropdown. For MultiSelect this feature would definitely require that user can clear the selected list by clicking on 'x' button.

https://primefaces.org/primereact/showcase/#/multiselect

Please tell us about your environment:

• React version:
  16.13.1

• PrimeReact version:
  4.2.1

• Browser: [all | Chrome XX | Firefox XX | IE XX | Safari XX | Mobile Chrome XX | Android X.X Web Browser | iOS XX Safari | iOS XX UIWebView | iOS XX WKWebView ]

• Language: [all | TypeScript X.X | ES6/7 | ES5]

[SadiaWaleem]

Thanks for implementing this. Thats the must need feature in multiselect.