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.

gui/dialogs/ProgressDialog.h

@@ -35,6 +35,7 @@ public:
 	void updateSize();
 
 	int exec(ProgressProvider *task);
+	void setSkipButton(bool present, QString label = QString());
 
 	ProgressProvider *getTask();
 
@@ -47,7 +48,10 @@ slots:
 	void changeStatus(const QString &status);
 	void changeProgress(qint64 current, qint64 total);
 
-signals:
+	
+private
+slots:
+	void on_skipButton_clicked(bool checked);
 
 protected:
 	virtual void keyPressEvent(QKeyEvent *e);