Qt: small header changes / fixes

- ensure header inclusion guard is named after the header file
- add missing comments at the end of some inclusion guards
- add a small Qt5 compatibility fix in macdockiconhandler.h

src/qt/macdockiconhandler.h

@@ -1,7 +1,7 @@
 #ifndef MACDOCKICONHANDLER_H
 #define MACDOCKICONHANDLER_H
 
-#include <QtCore/QObject>
+#include <QObject>
 
 class QMenu;
 class QIcon;