properly implement displayNamesInGroup in Core and LDAP, fixes #1689

what happens here: displayNamesInGroup was not declared necessary in OC_Group_Interface (though has a good implemtation in OC_Group_Backend). Because of this and not to break other apps that may implement the Interface at this point of time, i marked it optional using implementsAction(). The fallback behaviour in OC_Group is use internal name as display name, like OC_Group_Backend does.

To work well, properly and expected the functionality was added for the LDAP backend, too.

Reviewers please, especially but not only @schiesbn