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/net/NetJob.h

@@ -94,6 +94,8 @@ signals:
 public
 slots:
 	virtual void start();
+	// FIXME: implement
+	virtual void abort() {};
 private
 slots:
 	void partProgress(int index, qint64 bytesReceived, qint64 bytesTotal);