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/overviewpage.cpp

@@ -5,14 +5,14 @@
 #include "overviewpage.h"
 #include "ui_overviewpage.h"
 
-#include "clientmodel.h"
-#include "walletmodel.h"
 #include "bitcoinunits.h"
-#include "optionsmodel.h"
-#include "transactiontablemodel.h"
-#include "transactionfilterproxy.h"
-#include "guiutil.h"
+#include "clientmodel.h"
 #include "guiconstants.h"
+#include "guiutil.h"
+#include "optionsmodel.h"
+#include "transactionfilterproxy.h"
+#include "transactiontablemodel.h"
+#include "walletmodel.h"
 
 #include <QAbstractItemDelegate>
 #include <QPainter>