Add unwrapElement method to UpcastWriter

[msamsel]

Suggested merge commit message (convention)

Other: Add unwrapElement method to UpcastWriter.

---

Additional information

PR extracted from ckeditor/ckeditor5#2499`

[coveralls]

Coverage remained the same at 100.0% when pulling 61cf87f on t/paste-from-office/69 into 04858be on master.

[jodator]

The method should be similar to the writer.unwrap() from model writer.

[jodator]

The method should not return true/false - we don't need that.

Check the model's writer.unwrap() how we handle this:

ckeditor5-engine/src/model/writer.js

Lines 851 to 861 in ad9159c

	if ( element.parent === null ) {
	/**
	* Trying to unwrap an element which has no parent.
	*
	* @error writer-unwrap-element-no-parent
	*/
	throw new CKEditorError( 'writer-unwrap-element-no-parent: Trying to unwrap an element which has no parent.', this );
	}
	this.move( Range._createIn( element ), this.createPositionAfter( element ) );
	this.remove( element );

[jodator]

After some clarification from @msamsel I see that some methods of upcast writer return true/false in similar scenarios but TBH I don't see why. So I'm for not returning anything but adding a if( element.parent ) check to not perform unwrap on detached elements.

[jodator]

Now it's OK :)

[oskarwrobel]

Just for my curiosity. Why not just unwrap? Other methods from this class (like remove, replace, rename) don't have Element suffix.

[msamsel]

I've followed by this explanation:
ckeditor/ckeditor5-paste-from-office#72 (comment)