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/aboutdialog.cpp

@@ -6,6 +6,7 @@
 #include "ui_aboutdialog.h"
 
 #include "clientmodel.h"
+
 #include "clientversion.h"
 
 AboutDialog::AboutDialog(QWidget *parent) :