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/bitcoinstrings.cpp

@@ -1,4 +1,7 @@
+
+
 #include <QtGlobal>
+
 // Automatically generated by extract_strings.py
 #ifdef __GNUC__
 #define UNUSED __attribute__((unused))
@@ -221,4 +224,4 @@ QT_TRANSLATE_NOOP("bitcoin-core", "Warning"),
 QT_TRANSLATE_NOOP("bitcoin-core", "Warning: This version is obsolete, upgrade required!"),
 QT_TRANSLATE_NOOP("bitcoin-core", "You need to rebuild the database using -reindex to change -txindex"),
 QT_TRANSLATE_NOOP("bitcoin-core", "wallet.dat corrupt, salvage failed"),
-};
+};