[D&D] From outside into nested editable not working

[Szymon Cofalik]

Browser: Opera, maybe others. Clasic editor only.

D&D from outside source into nested editable is possible only if you the nested editable is empty or you had focus in it before selecting something from outside source.

Steps to reproduce:

1. Create image with caption (image2) and add a caption.
2. Move focus outside of nested editable (caption field).
3. Select some content from outside of editor.
4. Try to D&D it into caption.

Expected result: Selected text has been copied into caption.

Result: Test has been copied at the beginning of the editor.

You can test it here: ​http://tests.ckeditor.dev:10450/tests/plugins/clipboard/manual/draganddrop