TreeViewDelegate should support icon

the contentItem of TreeViewDelegate is Label (Material style), which is IconLabel in the base ItemDelegate.

whose result is the TreeViewDelegate not support icon (only a indicator).

I think it's better to use IconLabel like ItemDelegate, i.e. indicator and icon can exist at the same time (like windows file explorere).

Considering Editing capability, maybe we can separate it as RO/RW versions if we can't implement both editing and icon.