Can't tab through table inside a widget

[aske-cph]

Type: BUG

Original tracker issue: https://dev.ckeditor.com/ticket/14837

Steps to reproduce
Create a widget which allows inserting table inside editable. You can modify simple box.
Insert a table inside the widget.
Click inside first table cell and press tab

Expected result
Focus is moved to next table cell like in case of table not inserted into widget.

Actual result
Focus is moved to next widget or out of the editor.

Other details (latest : Chrome, Firefox, Safari , OSX Sierra, CKEditor 4.7.1)