Fire the change event after uploading is done

[Comandeer]

What is the purpose of this pull request?

Bug fix

Does your PR contain necessary tests?

All patches that change the editor code must include tests. You can always read more
on PR testing,
how to set the testing environment and
how to create tests
in the official CKEditor documentation.

This PR contains

• Unit tests
• Manual tests

Did you follow the CKEditor 4 code style guide?

Your code should follow the guidelines from the CKEditor 4 code style guide which helps keep the entire codebase consistent.

• PR is consistent with the code style guide

What is the proposed changelog entry for this pull request?

* [#5414](/~https://github.com/ckeditor/ckeditor4/issues/5414): Fixed: File and image uploaders based on the [Upload Widget plugin](https://ckeditor.com/cke4/addon/uploadwidget) and [Easy Image plugin ](https://ckeditor.com/cke4/addon/easyimage)didn't fire the [`change` event](https://ckeditor.com/docs/ckeditor4/latest/api/CKEDITOR_editor.html#event-change) upon finishing upload, resulting in passing incorrect data in form controls for integration frameworks, like [Reactive forms in Angular](https://angular.io/guide/reactive-forms).

What changes did you make?

In the case of easyimage, I've added firing of the change event at the end of the uploadDone listener.

In the case of uploadwidget and its dependents, I've added firing of the change event at the end of its replaceWith() method.

Which issues does your PR resolve?

Closes #5414.

[KarolDawidziuk]

LGTM! 🎉

@jacekbogdanski could you make an additional sanity check if everything is good?