Give the developers a means to enable/disable "PreventDefault" on KeyDown of UITreeNode

[marcodafonseca]

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

[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

Plunkr Case (Bug Reports)
https://stackblitz.com/edit/github-rub4ka

Current behavior
Arrow keys are only supported for navigating around the tree irrespective of whether a node may be editable or not. UITreeNode.OnKeyDown functionality fires preventDefault() method which stops the keypress event from propagating down to the input element

Expected behavior
If a user is editing a node's name they should be to navigate through the letters with the arrow keys like in any other application/website

Minimal reproduction of the problem with instructions
Minimal reproduction steps:
Add a p-tree element to a web-page
Make a node editable
Try navigate the letters of the editable node via the keyboard
https://stackblitz.com/edit/github-rub4ka

What is the motivation / use case for changing the behavior?
Improves usability for users

Please tell us about your environment:
Windows 10
Visual Studio

• Angular version: 5.X
  Angular 6.x

• PrimeNG version: 5.X
  PrimeNg 6.x

• 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 ]
  Any

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

• Node (for AoT issues): node --version =
  9.2.0

[mertsincan]

Duplicate of #8039