Create a TreeViewDelegate that can be more easily styled from the platform / app. The idea is that you can then do:

TreeView {
    delegate: TreeViewDelegate {
        TreeView.isExpanded: true
        indicator: Text{ text: TreeView.isExpanded ? "-" : "+" }
        icon: TreeView.isTreeColumn ? model.icon : null
        text: model.display
        alternateRows: false
    }
} 

But should it use one delegate for the tree, and another for the other columns? Or use TreeView.isTreeColumn?

Should TreeViewDelegate set icon and text from the model by default?