Update assignment client to close on WM_CLOSE message on Windows

StackManagerQt sends a WM_CLOSE when closing a process, but by default they are not handled - this adds handling for those messages.
This is a really good point, thank you for explaining it.  It would seem that this code should either be moved to a custom QApplication that implements this OS-specific behavior and can be derived from, or moved to a utility function.

Thoughts?  And where would be the right place in the repository to put it?  Thinking about other code I've written for this project that I would have considered library code, mainly Qt/GUI related, it would be nice to have a place in `libraries/...` for that sort of thing.

Thanks!