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

@@ -281,6 +281,7 @@ int main(int argc, char *argv[])
 #endif
                 app.exec();
 
+                window.hide();
                 window.setClientModel(0);
                 window.setWalletModel(0);
                 guiref = 0;