online/offline support, more bugfixes

I've started making use of the new online/offline connection status.When downloading or sending messages, Columba automatically switches to online state. If in offline status, Columba won't automatically check for new messages.Fixed bugs:- #966413: mimetype viewer doesn't remember apps- fixed classcast exception, actions are enabled/disabled correctly again in addressbook- group dialog didn't show up in addressbook- #968740, can't choose folder in filter dialog, if treemodel is not sorted