Add account page cannot be navigated with tab key

[doniks]

Reproduction:

- Nexus 7
- external keyboard and mouse plugged in
- Go to System Settings
- Accounts
- Add account
- Ubuntu One
=> User name field does not have keyboard focus
I would expect the first text field (user name) to have keyboard focus)

- click the user name field
- enter user name
- hit tab key
=> Nothing happens
I would expect that I can navigate through the relevant UI elements (user name, password, link, cancel button, sign in button)

- hit Esc key
=> nothing happens
I would expect this cancels the form

- click in the password field
- hit tab key
=> keyboard focus jumps back to the user name field
Strange.