Finish preliminary offline support

* ProgressProvider now has an abort() call
* Abort button support added to the progress dialog
* YggdrasilTask and MojangAccount adapted to support abort

YggdrasilTask will time out after 10 seconds of no network activity, or when the user pushes the Play Offline button.
In offline mode, all instance update tasks are skipped! This will need further work.

logic/OneSixInstance.cpp

@@ -37,9 +37,9 @@ OneSixInstance::OneSixInstance(const QString &rootDir, SettingsObject *setting_o
 	reloadFullVersion();
 }
 
-Task *OneSixInstance::doUpdate(bool prepare_for_launch)
+std::shared_ptr<Task> OneSixInstance::doUpdate(bool prepare_for_launch)
 {
-	return new OneSixUpdate(this, prepare_for_launch);
+	return std::shared_ptr<Task> (new OneSixUpdate(this, prepare_for_launch));
 }
 
 QString replaceTokensIn(QString text, QMap<QString, QString> with)