Expose Dialog icons for easier customization

[jc-white]

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Current behavior
p-dialog header buttons (maximize, close, etc) use hardcoded primeicons and there is no way to use an alternative such as font-awesome. I would like to have consistent icons and not have to install primeicons in addition to font-awesome.

Expected behavior
Would like to be able to override these icons with another library via component inputs. Currently I have to extend the component and override the template, which seems like overkill for simply overriding an icon.

Minimal reproduction of the problem with instructions

What is the motivation / use case for changing the behavior?

Please tell us about your environment:

• Angular version: 5.X
  6.x

• PrimeNG version: 5.X
  6.0.1

• 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]

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

[cagataycivici]

You can override them with css but sure we can expose them as props.

[jc-white]

Perfect, thanks @cagataycivici!