todoist: add provider for inital todoist-accounts

This patch calls gtd_plugin_todoist_account_added on
the accounts that were already present, once goa_client
is finished loading. After calling account_added,
every account is associated with a provider and
todoist provider is initalised.

This is done to make sure to load those accounts which were
already added before the goa_client was loaded.

https://bugzilla.gnome.org/show_bug.cgi?id=772278