[nemo-qml-plugins] Allow QStringList account configuration values

The latest libaccounts-qt allows QStringList configuration values to
be used during signon.  This is useful, for example, if you wish to
request multiple scopes (permissions) during oauth2 signon.
This commit relaxes the type checking to allow QStringList values to
be passed in.

It also fixes some minor issues with account models relating to the
account display name.