refactor: Move port mapping code to its own module

This commit does not change behavior.

src/qt/optionsmodel.cpp

@@ -13,11 +13,12 @@
 #include <qt/guiutil.h>
 
 #include <interfaces/node.h>
-#include <validation.h> // For DEFAULT_SCRIPTCHECK_THREADS
+#include <mapport.h>
 #include <net.h>
 #include <netbase.h>
-#include <txdb.h> // for -dbcache defaults
+#include <txdb.h>       // for -dbcache defaults
 #include <util/string.h>
+#include <validation.h> // For DEFAULT_SCRIPTCHECK_THREADS
 
 #include <QDebug>
 #include <QSettings>