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.
Hrm... good question.  Looking at our libraries the only place it might make sense to add it would be in the "shared" lib.  There is a lot of unrelated stuff there now, but I expect some of it will eventually be moved out into new libs, such as the physics/shape stuff.  I'd rather have to sort the shared lib later than to remove OS specific stuff out of multiple applications.

So you could make a class (ShutdownEventListener?) that implements QAbstractNativeEventListener, only supports Windows at the moment, and which catches the WM_CLOSE command.  The DomainServer and AssignmentClient classes could have an instance of this class rather than being one.

That's my first idea.  There's probably another way to do it.