Hide UI immediately after leaving the main loop.

Prevents it from seeming to hang during shutdown if shutdown is triggered while the window is open.

src/qt/bitcoin.cpp

@@ -205,6 +205,7 @@ int main(int argc, char *argv[])
 
                 app.exec();
 
+                window.hide();
                 guiref = 0;
             }
             Shutdown(NULL);