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.h

@@ -7,13 +7,14 @@
 
 #include <QWidget>
 
+class ClientModel;
+class TransactionFilterProxy;
+class TxViewDelegate;
+class WalletModel;
+
 namespace Ui {
     class OverviewPage;
 }
-class ClientModel;
-class WalletModel;
-class TxViewDelegate;
-class TransactionFilterProxy;
 
 QT_BEGIN_NAMESPACE
 class QModelIndex;