Implement detached threads (#88)

The `AsyncTerm` thread is never joined but its handle is immediately closed. This works fine on Windows, but causes issues on other platforms where threads need to be explicitly detached if never joined (pthread, wiiu).

This adds a new `PltDetachThread` function, which can be used instead of `PltCloseThread` to explictly detach a thread, requiring it to no longer be closed.

src/PlatformThreads.h

@@ -66,6 +66,7 @@ void PltCloseThread(PLT_THREAD* thread);
 void PltInterruptThread(PLT_THREAD* thread);
 bool PltIsThreadInterrupted(PLT_THREAD* thread);
 void PltJoinThread(PLT_THREAD* thread);
+void PltDetachThread(PLT_THREAD* thread);
 
 int PltCreateEvent(PLT_EVENT* event);
 void PltCloseEvent(PLT_EVENT* event);