QFileSystemModel: add a symbolic constant for the number of columns

There are other literal 4's used in the implementation, so
disambiguate.

Change-Id: I5b0e4ac0018f9d5734e3de8b4f75259e175b23d3
Reviewed-by: David Faure <david.faure@kdab.com>

src/widgets/dialogs/qfilesystemmodel.cpp

@@ -643,7 +643,7 @@ int QFileSystemModel::rowCount(const QModelIndex &parent) const
 */
 int QFileSystemModel::columnCount(const QModelIndex &parent) const
 {
-    return (parent.column() > 0) ? 0 : 4;
+    return (parent.column() > 0) ? 0 : QFileSystemModelPrivate::NumColumns;
 }
 
 /*!

src/widgets/dialogs/qfilesystemmodel_p.h

@@ -70,6 +70,8 @@ class Q_AUTOTEST_EXPORT QFileSystemModelPrivate : public QAbstractItemModelPriva
     Q_DECLARE_PUBLIC(QFileSystemModel)
 
 public:
+    enum { NumColumns = 4 };
+
     class QFileSystemNode
     {
     public: