Add cancel dialog event listener & Export FeedbackService, FeedbackDirective

[workfel]

• I need service to force re-render the screenshot, because before set screen image i need to hide all private user information.

[artar94]

@workfel I think it would be great to make another service specifically for export.
It should have only one method feedback(): Observable<Feedback>, which, if successful, returns Feedback, and if canceled, returns an error.
This will allow you to use this library with any buttons and directives, not just the built-in one, and it will not force the user to deal with the internal organization of the initScreenshotCanvas, setFeedback, openFeedbackDialog, etc. methods.

[workfel]

ok, but why don't juste export the current Service ?

[artar94]

I have nothing against it, but I would recommend at least renaming the current FeedbackService to FeedbackInternalService so that the new service has a user-friendly name.

[workfel]

Ok, i just updated the PR