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/optionsdialog.cpp

@@ -11,9 +11,10 @@
 
 #include "bitcoinunits.h"
 #include "monitoreddatamapper.h"
-#include "netbase.h"
 #include "optionsmodel.h"
 
+#include "netbase.h"
+
 #include <QDir>
 #include <QIntValidator>
 #include <QLocale>