QTree - expandedIcon / collapsedIcon

[Oltioff]

Hi,
I don't think prop Icon is useful as it is.
As far as I can see, there's a rotation once the node is expanded.

I think it would be more useful to have 2 differente props: expandedIcon and collapsedIcon and render the proper icon based on node expansion status.

I'll try on my own to create a pull request