include keys from user's .ssh directory in required key dropdown

The required-key dropdown list on the Add/Update Tunnel Dialog could also include a list of keys that are in the user's .ssh directory but not yet in the current key configuration. If the selected key is not yet in the configuration, then it would be added to the configuration when the [OK] button is clicked. In this way, a user could generate the list of keys being managed entirely from the Add/Update Tunnel Dialog.