Some header inclusion clean-ups.

o Include what is directly used, nothing more.
 o Don't rely on transitive inclusion.
 o Order inclusions as: 1) own, 2) Qt, 3) others.
 o Order inclusions alphabetically.
 o Use forward declarations where possible.
 o Blank line between include guard and #include directives.

Tested on Qt 4.8.6 and Qt 5.4.1.

This incidentally fixes "invalid use of incomplete type ‘class
QStringList’" which I got in pack_p.cpp with Qt 4.8.6.

src/private/pack_p.cpp

@@ -1,7 +1,13 @@
 #include "pack_p.h"
 #include "private/sysdep.h"
-#include <limits>
+
+#include <QByteArray>
 #include <QDebug>
+#include <QMapIterator>
+#include <QString>
+#include <QStringList>
+
+#include <limits>
 
 QHash<QMetaType::Type, MsgPackPrivate::packer_t> MsgPackPrivate::user_packers;
 bool MsgPackPrivate::compatibilityMode = false;