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/walletmodeltransaction.h

@@ -7,8 +7,14 @@
 
 #include "walletmodel.h"
 
+#include <QObject>
+
 class SendCoinsRecipient;
 
+class CReserveKey;
+class CWallet;
+class CWalletTx;
+
 /** Data model for a walletmodel transaction. */
 class WalletModelTransaction
 {