qt: Avoid splash-screen related memory leak

Make splash screen queue its own deletion when it receives the finished
command, instead of relying on WA_DeleteOnClose which doesn't work under
these circumstances.

src/qt/splashscreen.cpp

@@ -134,6 +134,7 @@ void SplashScreen::slotFinish(QWidget *mainWin)
 {
     Q_UNUSED(mainWin);
     hide();
+    deleteLater(); // No more need for this
 }
 
 static void InitMessage(SplashScreen *splash, const std::string &message)