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/splashscreen.cpp

@@ -14,8 +14,9 @@
 #endif
 
 #include <QApplication>
-#include <QPainter>
+#include <QCloseEvent>
 #include <QDesktopWidget>
+#include <QPainter>
 
 SplashScreen::SplashScreen(Qt::WindowFlags f, bool isTestNet) :
     QWidget(0, f), curAlignment(0)
@@ -113,7 +114,6 @@ SplashScreen::~SplashScreen()
 void SplashScreen::slotFinish(QWidget *mainWin)
 {
     hide();
-    deleteLater();
 }
 
 static void InitMessage(SplashScreen *splash, const std::string &message)
@@ -175,3 +175,8 @@ void SplashScreen::paintEvent(QPaintEvent *event)
     painter.drawText(r, curAlignment, curMessage);
 }
 
+void SplashScreen::closeEvent(QCloseEvent *event)
+{
+    event->ignore();
+}
+