daemon: Allow account ID to be provided for new accounts

org.gnome.OnlineAccounts.Manager.AddAccount will now check for an 'Id'
key in the 'details' argument, and use that as the account ID (aka the
group name in the accounts.conf key file). If absent, an account ID is
generated same as before.

https://bugzilla.gnome.org/show_bug.cgi?id=737721