title: Extending QFileSystemModel by a bool variable
body: <p>I would like to extend the QFileSystemModel. I would like to have a custom role <code>IsSelectedRole</code> where I can store whether a bool value which stores whether a file got selected in a QML <code>TreeView</code>. More precisely, I don't know how to set the <code>setData</code> and <code>data</code> function of my <code>QFileSystemModel</code> derived class, like where to store the data. I guess making my own tree of <code>bool</code> variables should work, but I hope that there is an easier way.</p>