Cannot navigate caption button language menu with accessibility

[robertmausser]

Caption button is not accessible through tab/ arrow keys. When user hits enter on it then nothing happens.

Caption button should be accessible through tab/ arrow keys. When user hits enter on it then the button should expand.

If caption button will not be accessible then screen reader and keyboard user will not be able to access its functionality.

https://www.screencast.com/t/mLvG21vtC

Provide proper tab indexing and aria role to caption button.

[swashbuck]

We can review this again as part of #299

[swashbuck]

Needs reviewing as part of #299