Tab key based navigation is broken in iframedialog plugin.

[Jakub Ś]

Problem is similar to #9611.

It isn't possible to jump (with tabIndex) through elements that are inside iframedialog plugin.
Problem can be reproduced in all browsers.

NOTES:
Firefox, Webkit and Blink - When you click on page inside iframe you can jump through elements but when you reach Cancel button there is no coming back.
IE - Result is the same (buttons grab focus) but there is one note to make.
Focus works a little different here even on native HTML page. Most of the time, you need to click inside input area so that you can tab through elements. When you click on page then (most of the time and it depends where you click) you will focus select element or nothing.

[kkrzton]

There is a dedicated mechanism for managing focus in dialogs. When component is added, it is registered as focusable so it can be focus later. Iframe is added via addIframe method which does not register it as focusable.

[kkrzton]

Registering iframe as focusable element solves problem with focusing it so elements inside can be navigated via tab key, but there are still few issues:

• In Chrome there is one unnecessary tab press which focuses just iframe - should focus first element inside iframe (next tab press, focuses first focusable element inside iframe).
• In FF when refocusing iframe - last previously focused element in iframe gets focused instead of first focusable element.
• In IE after last button in dialog is focused, next tab press moves focus on iframe element which gets stuck there and won't move any further.

Also I think tab navigation inside iframe should not be handled by focus management mechanism (at the moment it isn't) which means leaving iframe using tab key must be handled separately.