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/transactionfilterproxy.h

@@ -5,8 +5,8 @@
 #ifndef TRANSACTIONFILTERPROXY_H
 #define TRANSACTIONFILTERPROXY_H
 
-#include <QSortFilterProxyModel>
 #include <QDateTime>
+#include <QSortFilterProxyModel>
 
 /** Filter the transaction list according to pre-specified rules. */
 class TransactionFilterProxy : public QSortFilterProxyModel