refactor, qt: Add SHUTDOWN_POLLING_DELAY constant

A named constant is better for the code readability. Also it could be
reused in an alternative GUI implementation (e.g., QML-based).

src/qt/bitcoin.cpp

@@ -414,7 +414,7 @@ void BitcoinApplication::initializeResult(bool success, interfaces::BlockAndHead
             QTimer::singleShot(100ms, paymentServer, &PaymentServer::uiReady);
         }
 #endif
-        pollShutdownTimer->start(200ms);
+        pollShutdownTimer->start(SHUTDOWN_POLLING_DELAY);
     } else {
         Q_EMIT splashFinished(); // Make sure splash screen doesn't stick around during shutdown
         requestShutdown();

src/qt/guiconstants.h

@@ -13,6 +13,9 @@ using namespace std::chrono_literals;
 /* A delay between model updates */
 static constexpr auto MODEL_UPDATE_DELAY{250ms};
 
+/* A delay between shutdown pollings */
+static constexpr auto SHUTDOWN_POLLING_DELAY{200ms};
+
 /* AskPassphraseDialog -- Maximum passphrase length */
 static const int MAX_PASSPHRASE_SIZE = 1024;