[Qt] catch Windows shutdown events while client is running

- prevents unsafe shutdowns on Windows, which is known to be
  able to cause problems with wallet.dat
- if a users ends a Windows session, this will initiate a client shutdown
  and show a Windows dialog, that tells the user what is going on (for
  Windows Vista and higher it will even show a reason for blocking the
  Windows session end)

src/qt/winshutdownmonitor.h

@@ -0,0 +1,29 @@
+// Copyright (c) 2014 The Bitcoin developers
+// Distributed under the MIT/X11 software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#ifndef WINSHUTDOWNMONITOR_H
+#define WINSHUTDOWNMONITOR_H
+
+#ifdef WIN32
+#include <QByteArray>
+#include <QString>
+
+#if QT_VERSION >= 0x050000
+#include <windef.h> // for HWND
+
+#include <QAbstractNativeEventFilter>
+
+class WinShutdownMonitor : public QAbstractNativeEventFilter
+{
+public:
+    /** Implements QAbstractNativeEventFilter interface for processing Windows messages */
+    bool nativeEventFilter(const QByteArray &eventType, void *pMessage, long *pnResult);
+
+    /** Register the reason for blocking shutdown on Windows to allow clean client exit */
+    static void registerShutdownBlockReason(const QString& strReason, const HWND& mainWinId);
+};
+#endif
+#endif
+
+#endif // WINSHUTDOWNMONITOR_H