Move the plugin examples to subdir of pluginhowto.

doc/pluginhowto/examples/dirmodelplugin/filesystemmodel.cpp

@@ -0,0 +1,20 @@
+#include "filesystemmodel.h"
+
+FileSystemModel::FileSystemModel(QObject* parent)
+    :QFileSystemModel(parent)
+{
+
+}
+
+FileSystemModel::~FileSystemModel()
+{
+
+}
+
+int FileSystemModel::columnCount(const QModelIndex &parent ) const
+{
+    Q_UNUSED(parent)
+    return 1;
+}
+
+