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/MainWindow.h

@@ -54,6 +54,7 @@ public:
 
 	void checkInstancePathForProblems();
 
+	void updatesAllowedChanged(bool allowed);
 signals:
 	void isClosing();