Fire change event after upload completes

[rrg92]

What is the purpose of this pull request?

This allows user to get the updated data after image is uploaded.

Today, there is a bug, that after the upload completes, the call to getData() returns an old HTML version (without the image tag updated...)

This solve the issue #1180

Does your PR contain necessary tests?

This PR contains

• [ ] Unit tests
• [x] Manual tests

Test:

• Create a simplest editor with image and imageupload pluging
• Configure upload to some valid upload server
• Add the following event listener:

editor.on('change',function(e){  console.log(e.editor.getDate())  })

• Paste some image into editor (and observe)
• Before this fix, 'change' is not fired
• After this fix, 'change' is fired and user can handle new data.

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.

• [x] PR is consistent with the code style guide

What is the proposed changelog entry for this pull request?

* [#1180](https://github.com/ckeditor/ckeditor4/issues/1180): Fix bug when uploading pasting data and change event not fired correctly.

What changes did you make?

Just added a call to fire method of editor instance, firing the "change" event.

Which issues does your PR resolve?

Closes #1180