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/qt/bitcoin.cpp

@@ -290,6 +290,7 @@ int main(int argc, char *argv[])
                 window.setWalletModel(0);
                 guiref = 0;
             }
+            // Shutdown the core and it's threads, but don't exit Bitcoin-Qt here
             Shutdown(NULL);
         }
         else