Snackbar add keyboard functions when theres a button

[djbuckley]

Keyboard

Add the button on the snackbar to the next place in the tab index, if a button is shown.

• Size 8

[m4ttb13]

Focus moves to the action button when tabbed to it - discussion to be had to determine if it should auto focus onto this button or leave as it is.

[m4ttb13]

Going to sign off as it is because it is accessible, and the nature of the snackbar itself means it is not designed with accessibility in mind

[m4ttb13]

mui#4116 details as far as I got. setting focus seems to move the visual but not actual focus onto the snackbar. Could not move past this.