Add log entry when opening a file via Qt event

This helps debugging CURA-730, and might also help debugging similar issues in the future. It's a user-triggered event so this warrants an info-level log entry.

Contributes to issue CURA-730.

cura/CuraApplication.py

@@ -207,6 +207,7 @@ class CuraApplication(QtApplication):
     #   Handle Qt events
     def event(self, event):
         if event.type() == QEvent.FileOpen:
+            Logger.log("i", "File open via Qt event: %s", event.file())
             self._openFile(event.file())
 
         return super().event(event)