Cleanup code using forward declarations.

Use misc methods of avoiding unnecesary header includes.
Replace int typedefs with int##_t from stdint.h.
Replace PRI64[xdu] with PRI[xdu]64 from inttypes.h.
Normalize QT_VERSION ifs where possible.
Resolve some indirect dependencies as direct ones.
Remove extern declarations from .cpp files.

src/qt/clientmodel.cpp

@@ -5,21 +5,21 @@
 #include "clientmodel.h"
 
 #include "guiconstants.h"
-#include "optionsmodel.h"
-#include "addresstablemodel.h"
-#include "transactiontablemodel.h"
 
-#include "chainparams.h"
 #include "alert.h"
-#include "main.h"
+#include "chainparams.h"
 #include "checkpoints.h"
+#include "main.h"
+#include "net.h"
 #include "ui_interface.h"
 
-#include <QDateTime>
-#include <QTimer>
-#include <QDebug>
+#include <stdint.h>
 
-static const int64 nClientStartupTime = GetTime();
+#include <QDateTime>
+#include <QDebug>
+#include <QTimer>
+
+static const int64_t nClientStartupTime = GetTime();
 
 ClientModel::ClientModel(OptionsModel *optionsModel, QObject *parent) :
     QObject(parent), optionsModel(optionsModel),