Pop up message box on completion

[GenevieveBuckley]

I'd like to be able to have a message box pop up at the end of some long running computation to tell the user it is complete.

I'm aware you can do something sort of similar with the result_widget line edit box, but since it displays "None" before any value is returned it seems too likely to confuse users into thinking they need to enter a value into it before they can start the program.

I'm not sure whether I can just create a Qt message box of my own for this, but it seems cleaner to have it connected to the magicgui application class somehow.