GH-1874 do not allow updating while an instance is running

This is a nasty hack. Proper solution will require moving all
update related functionality out of the main window.

Running instances and updating should be mutually exclusive.

application/MultiMC.h

@@ -151,6 +151,11 @@ public:
 		return m_runningInstances;
 	}
 
+	bool updatesAreAllowed();
+
+signals:
+	void updateAllowedChanged(bool status);
+
 public slots:
 	bool launch(InstancePtr instance, bool online = true, BaseProfilerFactory *profiler = nullptr);
 	bool kill(InstancePtr instance);
@@ -186,6 +191,11 @@ private:
 	// sets the fatal error message and m_status to Failed.
 	void showFatalErrorMessage(const QString & title, const QString & content);
 
+private:
+	void addRunningInstance();
+	void subRunningInstance();
+	bool shouldExitNow() const;
+
 private:
 	QDateTime startTime;