introduce a new StartShutdown() function, which starts a thread with Shutdown() if no GUI is used and calls uiInterface.QueueShutdown() if a GUI is used / all direct uiInterface.QueueShutdown() calls are replaced with Shutdown() - this ensures a clean GUI shutdown, even when catching a SIGTERM and allows the BitcoinGUI destructor to get called (which fixes a tray-icon issue and keeps the tray-icon until Bitcoin-Qt exits)

src/net.cpp

@@ -1658,7 +1658,7 @@ void ThreadMessageHandler2(void* parg)
         vnThreadsRunning[THREAD_MESSAGEHANDLER]--;
         Sleep(100);
         if (fRequestShutdown)
-            Shutdown(NULL);
+            StartShutdown();
         vnThreadsRunning[THREAD_MESSAGEHANDLER]++;
         if (fShutdown)
             return;