Reimplement TestConnection() background class

At present, the check connection button uses the duplicity backend class list() function in order to check if the connection is valid. If the user is able to connect, this will happen very quickly. Unfortunately, if the user is not able to connect, then duplicity will attempt to reconnect six or seven times. This is an extremely long-running backend process (though it is implemented as a background thread, so it does not tie up the userinterface).

Code should be contributed back to duplicity which enables a CheckConnection or testConnection method.