Focus trap inside dialog tabs

[Marek Lewandowski]

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

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;
   }