This fixes a lot of the issues from #2746 by making the dialog inherit from our actual ModalDialog which now has tab focus trapping.
The Captions Settings dialog has some accessibility issues:
- Field labels and fields are not explicitly associated
- Keyboard focus does not move into the dialog when it is opened
- Keyboard focus is not trapped inside the dialog while it is open
- Keyboard focus does not return to the control which opened the dialog when it is closed
- The extent (top and bottom) of the dialog is not indicated to screen readers
- The dialog cannot be closed with the Esc key
- The meaning of '---' in the select fields is not clear
- The control to close the dialog is labeled "Done" rather than "Close"
- The purpose of the "Defaults" button may not be obvious, and its effect may not be apparent to screen reader users
- Focus highlighting (outline) on the Default and Done buttons is *very* hard to see
- Pressing the Done button doesn’t seem to have the same effect as pressing the Close (x) button; does it trigger the same focus movement
- This requirement to move it back to the triggering element is tricky, since clicking on that item in the CC menu dismisses the CC menu. I need to think about this a little more - either the menu should open again, or the focus should go to the main CC Menu Button
- The focus outline on the whole dialog goes too far to the left (all the way to the edge of the video window, not just to the edge of the dialog)
Fixes#2746.
