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/bitcoinunits.h

@@ -5,8 +5,8 @@
 #ifndef BITCOINUNITS_H
 #define BITCOINUNITS_H
 
-#include <QString>
 #include <QAbstractListModel>
+#include <QString>
 
 /** Bitcoin unit definitions. Encapsulates parsing and formatting
    and serves as list model for drop-down selection boxes.