qt: Make splash and shutdown window ignore close events

It's strange to be able to close these windows while there is work
in progress.

Also set Qt::WA_DeleteOnClose on both windows to make sure that they
are deleted eventually, no matter what happens.

src/qt/bitcoin.cpp

@@ -339,6 +339,9 @@ void BitcoinApplication::createWindow(bool isaTestNet)
 void BitcoinApplication::createSplashScreen(bool isaTestNet)
 {
     SplashScreen *splash = new SplashScreen(0, isaTestNet);
+    // 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);
     splash->show();
     connect(this, SIGNAL(splashFinished(QWidget*)), splash, SLOT(slotFinish(QWidget*)));
 }