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.h

@@ -7,12 +7,13 @@
 
 #include <QDialog>
 
+class MonitoredDataMapper;
+class OptionsModel;
+class QValidatedLineEdit;
+
 namespace Ui {
 class OptionsDialog;
 }
-class OptionsModel;
-class MonitoredDataMapper;
-class QValidatedLineEdit;
 
 /** Preferences dialog. */
 class OptionsDialog : public QDialog