Focus in tabs panel works in a weird way if there are some disabled tabs.

[Marek Lewandowski]

Disabled tabs should be focusable, but user must not be able to activate them. It is the same policy as for disabled toolbar buttons.

Consider following TC:

1. Open ​http://ckeditor.dev/plugins/image2/samples/image2.html
2. Place caret in an image caption.
3. Open link dialog.
4. Focus tab (the only enabled).
5. Press right arrow key.

Expected result:
You should be able to focus any tab despite it being disabled or not.

Current result:
Using left/right keys you can reach the disabled tabs, but you can't leave them then.

Additional info:

1. You might have hard times seeing what's focused if branch ​t/13027 was not yet merged. In such case add following CSS rule:

   a.cke_dialog_tab:focus
   {
   	background: #ebebeb;
   }