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/askpassphrasedialog.cpp

@@ -8,9 +8,11 @@
 #include "guiconstants.h"
 #include "walletmodel.h"
 
+#include "allocators.h"
+
+#include <QKeyEvent>
 #include <QMessageBox>
 #include <QPushButton>
-#include <QKeyEvent>
 
 AskPassphraseDialog::AskPassphraseDialog(Mode mode, QWidget *parent) :
     QDialog(parent),