Drag and drop element with id duplicate id

[Piotr Jasiun]

1. Create editor instance with the element with ID:

` <p>foo<a href="​http://ckeditor.com" id="link">bar</a></p> `

2. Drag part of the link (ex. letter "a"),
3. Drop it somewhere else (ex. after "fo", before "o").

Result: two elements with the same ID: ` <p>fo<a href="​http://ckeditor.com" id="link">a</a>o<a href="​http://ckeditor.com" id="link">br</a></p> `

Is should not be possible to duplicate ID by drag and drop. Paste event may check if there is element already element with that ID and remove "id" attribute if so.

On the other hand the same issue exists with the native drag and drop on Chrome.

[Jakub Ś]

This issue is related to and probably based on #6915 but of course valid.