[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.cpp

@@ -0,0 +1,57 @@
+// 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.
+
+#include "winshutdownmonitor.h"
+
+#if defined(Q_OS_WIN) && QT_VERSION >= 0x050000
+#include "init.h"
+
+#include <windows.h>
+
+#include <QDebug>
+
+// If we don't want a message to be processed by Qt, return true and set result to
+// the value that the window procedure should return. Otherwise return false.
+bool WinShutdownMonitor::nativeEventFilter(const QByteArray &eventType, void *pMessage, long *pnResult)
+{
+       Q_UNUSED(eventType);
+
+       MSG *pMsg = static_cast<MSG *>(pMessage);
+
+       switch(pMsg->message)
+       {
+           case WM_QUERYENDSESSION:
+           {
+               // Initiate a client shutdown after receiving a WM_QUERYENDSESSION and block
+               // Windows session end until we have finished client shutdown.
+               StartShutdown();
+               *pnResult = FALSE;
+               return true;
+           }
+
+           case WM_ENDSESSION:
+           {
+               *pnResult = FALSE;
+               return true;
+           }
+       }
+
+       return false;
+}
+
+void WinShutdownMonitor::registerShutdownBlockReason(const QString& strReason, const HWND& mainWinId)
+{
+    typedef BOOL (WINAPI *PSHUTDOWNBRCREATE)(HWND, LPCWSTR);
+    PSHUTDOWNBRCREATE shutdownBRCreate = (PSHUTDOWNBRCREATE)GetProcAddress(GetModuleHandleA("User32.dll"), "ShutdownBlockReasonCreate");
+    if (shutdownBRCreate == NULL) {
+        qDebug() << "registerShutdownBlockReason : GetProcAddress for ShutdownBlockReasonCreate failed";
+        return;
+    }
+
+    if (shutdownBRCreate(mainWinId, strReason.toStdWString().c_str()))
+        qDebug() << "registerShutdownBlockReason : Successfully registered: " + strReason;
+    else
+        qDebug() << "registerShutdownBlockReason : Failed to register: " + strReason;
+}
+#endif