Also accepts additional data to pre-seed the GOA dialog, in particular
the provider name (eg. 'google') and provider-specific data encoded in a
a GVariant dict ('a{sv}') to eg. pass cookies from an existing web
session (this is only available from DBus activation).
It can be run from the command line as:
$ gnome-control-center online-accounts add google
https://bugzilla.gnome.org/show_bug.cgi?id=694315
