Resolve the qt/guiutil <-> qt/optionsmodal CD

This pull request attempts to resolve the `qt/guiutil` <-> `qt/optionsmodel`
circular dependency.

The circular dependency is resolved by moving the `Intro::getDefaultDataDirectory`
member function to `GUIUtil::getDefaultDataDirectory`.

src/qt/guiutil.cpp

@@ -244,6 +244,11 @@ QList<QModelIndex> getEntryData(QAbstractItemView *view, int column)
     return view->selectionModel()->selectedRows(column);
 }
 
+QString getDefaultDataDirectory()
+{
+    return boostPathToQString(GetDefaultDataDir());
+}
+
 QString getSaveFileName(QWidget *parent, const QString &caption, const QString &dir,
     const QString &filter,
     QString *selectedSuffixOut)