feat(window): add component instance property to window ref

[katebatura]

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• I read and followed the CONTRIBUTING.md guide.
• I read and followed the New Feature Checklist guide.

Short description of what this resolves:

Closes #2367

Added componentInstance property to NbWindowRef contains the instance of the component opened into he window.

How to use:

// window component
...
onAdd = new EventEmitter();

onButtonClick() {
  this.onAdd.emit();
}
...

// parent component
...
constructor(private windowService: NbWindowService) {}

openWindow() {
  let windowRef<Component> = this.windowService.open(Component);

  const sub = windowRef.componentInstance.onAdd.subscribe(() => {
    // do something
  });
  windowRef.onClose().subscribe(() => {
    // unsubscribe onAdd
  });
}
...

[yggg]

componentInstance

[yggg]

Suggested change

	* Property `windowContentInstance` contains the instance of the component opened into the window.
	* Property `contentInstance` contains the instance of the component opened in the window.