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.h

@@ -7,10 +7,11 @@
 
 #include <QDialog>
 
+class WalletModel;
+
 namespace Ui {
     class AskPassphraseDialog;
 }
-class WalletModel;
 
 /** Multifunctional dialog to ask for passphrases. Used for encryption, unlocking, and changing the passphrase.
  */