This makes the behavior much more clear. You can get a tristate checkbox
just by setting the CheckStateRole to PartiallyChecked, no tristate flag needed.
The flag, on the other hand, enables the automatic-tristate behavior in
QTreeViews (and only there), hence the new name for it.
Change-Id: I18d292a8b8294c863eab806f3874d15dfb72556c
Reviewed-by: Jarek Kobus <jaroslaw.kobus@theqtcompany.com>