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/checkpoints.h

@@ -1,13 +1,14 @@
 // Copyright (c) 2009-2013 The Bitcoin developers
 // Distributed under the MIT/X11 software license, see the accompanying
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
 #ifndef BITCOIN_CHECKPOINT_H
 #define BITCOIN_CHECKPOINT_H
 
 #include <map>
 
-class uint256;
 class CBlockIndex;
+class uint256;
 
 /** Block-chain checkpoints are compiled-in sanity checks.
  * They are updated every release or three.