Filter inputs in component should have aria-label for accessibility

[paraskt]

If you have a PrimeNG PRO Support subscription please post your issue at;

https://pro.primefaces.org

where our team will respond within 4 business hours.

If you do not have a PrimeNG PRO Support subscription, fill-in the report below. Please note that
your issue will be added to the waiting list of community issues and will be reviewed on a first-come first-serve basis, as a result, the support team is unable to guarantee a specific schedule on when it will be reviewed. Thank you for your understanding.

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

[x] bug report => Search github for a similar issue or PR before submitting
[ ] 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

Plunkr Case (Bug Reports)
Please demonstrate your case at stackblitz by using the issue template below. Issues without a test case have much less possibility to be reviewd in detail and assisted.

https://stackblitz.com/github/primefaces/primeng-issue-template

Current behavior
Search textbox in p-dropdown does not have associated label, so the screen reader cant read that this is a search text box. People relying only on screen readers will not know what this textbox is for.

Expected behavior
Search textbox in p-dropdown must have associated label

Minimal reproduction of the problem with instructions

What is the motivation / use case for changing the behavior?
https://dequeuniversity.com/rules/axe/2.6/label
Adding effective form labels is absolutely necessary to make forms accessible. The purpose of form elements such as a checkboxes, radio buttons, input fields, etc. is often clear for sighted users, even if the form element is not programmatically labeled. This isn't usually the case for screen reader users. Adding a label to all form elements eliminates ambiguity and contributes to a more accessible product.

Please tell us about your environment:

• Angular version: 5.X
  5.1.3

• PrimeNG version: 5.X
  5.2.5

• 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 ]
  chrome, edge

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

• Node (for AoT issues): node --version =

[cagataycivici]

Let's expand it to dropdown, multiselect, listbox, picklist and orderlist as well by adding new ariaFilterLabel prop.