feat(window): window may return result

[katebatura]

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Short description of what this resolves:

Closes #2696

The window may return result through NbWindowRef. Calling component can receive this result with onClose stream of NbWindowRef.

How to use:

this.windowService.open(SomeComponent, {title: `Window`}).onClose.subscribe(result => {
    //do something...
});

Example is here: https://akveo.github.io/nebular/docs/components/window/overview#nbwindowservice