Ensure mutexes are always properly unlocked

Error exists with Qt4 and Qt5.  Qt5 generated helpful warnings that made
it easier to debug.

src/Gui/Splashscreen.cpp

@@ -134,7 +134,7 @@ public:
 
         splash->showMessage(msg.replace(QLatin1String("\n"), QString()), alignment, textColor);
         QMutex mutex;
-        mutex.lock();
+        QMutexLocker ml(&mutex);
         QWaitCondition().wait(&mutex, 50);
     }