New account dialog improvements
The new account dialog looks quite raw, i think it should:
- say "New account" instead of "Preferences" in the top bar
- use GtkEntry's placeholder text to suggest a "User ID" format (e.g. @foo:example.com)
- allow the user to alt-tab out of the "User ID" entry even when the user name is not complete (currently alt-tab does nothing when in the User ID field)
/cc @tobias.bernard so I don't suggest stuff that's not up to date anymore