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/bitcoinamountfield.cpp

@@ -4,15 +4,14 @@
 
 #include "bitcoinamountfield.h"
 
-#include "qvaluecombobox.h"
 #include "bitcoinunits.h"
 #include "guiconstants.h"
+#include "qvaluecombobox.h"
 
 #include <QApplication>
+#include <QDoubleSpinBox>
 #include <QHBoxLayout>
 #include <QKeyEvent>
-#include <QDoubleSpinBox>
-
 #include <qmath.h> // for qPow()
 
 BitcoinAmountField::BitcoinAmountField(QWidget *parent):