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/paymentrequestplus.h

@@ -5,13 +5,14 @@
 #ifndef PAYMENTREQUESTPLUS_H
 #define PAYMENTREQUESTPLUS_H
 
+#include "paymentrequest.pb.h"
+
+#include "base58.h"
+
 #include <QByteArray>
 #include <QList>
 #include <QString>
 
-#include "base58.h"
-#include "paymentrequest.pb.h"
-
 //
 // Wraps dumb protocol buffer paymentRequest
 // with extra methods