TreeViewDelegate: respect TreeView's rootIndex

The Qt Quick TreeView allows to set its root index so that only a part of the model is shown. For example, in a file tree, you could set the root index to the user's home directory, while the file system model is rooted at / (see the Qt File Explorer Example).

So far, the position of the branch indicators is computed based on the distance to the model's root, ignoring the TreeView's root index. This MR fixes that.

Before: After: