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.
My first idea would be to have a runtime OS check that disables logging.  Something like this:

```
if (OSUtils::isWindows()) {
    LogUtils::disableLogging();
}
```
The advantage: it is very clear to anybody reading the code what it is doing.  The disadvantages: you'd need to implement OSUtils.h and LogUtils.h, both of which would be very thin, this doesn't look much better than #ifdef OS stuff, and incurs a slight runtime cost which some developers might find objectionable on principle.

An alternative would be something like this:
```
LogUtils::init();
```
Initially the only thing LogUtils::init() would do is disable logging for Windows (inside an #ifdef) but it could eventually be enhanced to write to a file that was specified in preferences or even to syslog on supported platforms.  The disadvantages: it obfuscates what is actually going on, and someone might naively think they should add a LogUtils::init() to a future application that doesn't need the logs disabled on windows.  I guess this would be my preference for now -- I'm guessing that a featureful LogUtils would eventually be useful for systems that manage many server processes.