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/bitcoin.cpp

@@ -365,9 +365,8 @@ void BitcoinApplication::createWindow(const NetworkStyle *networkStyle)
 void BitcoinApplication::createSplashScreen(const NetworkStyle *networkStyle)
 {
     SplashScreen *splash = new SplashScreen(0, networkStyle);
-    // We don't hold a direct pointer to the splash screen after creation, so use
-    // Qt::WA_DeleteOnClose to make sure that the window will be deleted eventually.
-    splash->setAttribute(Qt::WA_DeleteOnClose);
+    // We don't hold a direct pointer to the splash screen after creation, but the splash
+    // screen will take care of deleting itself when slotFinish happens.
     splash->show();
     connect(this, SIGNAL(splashFinished(QWidget*)), splash, SLOT(slotFinish(QWidget*)));
 }

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)