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/version.h

@@ -5,6 +5,7 @@
 #define BITCOIN_VERSION_H
 
 #include "clientversion.h"
+
 #include <string>
 
 //