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 example of the "overuse of ifdef" anti-pattern.  We don't address it specifically in our coding standard and therefore doesn't block this PR.  Also, I don't think the hifi dev team has come to a consensus on the issue, but it is one of the things I look out for.

Specifically, whenever I see an #ifdef OPERATING_SYSTEM I ask myself: is this in library or application code?  Such checks are necessary when writing ported libraries however application code can avoid them by pushing the checks into the libraries and forcing the libraries to become abstraction layers that protect the application code from OS specific concerns.  The end result is application code that is "portable" and library code that has been "ported".

The needs of the assignment-client on Windows have revealed that our library abstractions are insufficient.  If there is an easy way to move this stuff into a library without too much trouble then we should do it, since other applications may eventually have the same need.  If it is too much work for now then we can punt it and fix it later.