treeview's delegate is destructed after rowsremoved, not rowsabouttoberemoved

the delgegate is deleted after model's signal rowsRemoved.

the model's signal rowsAboutToBeRemoved should trigger view to deleting delegates (like other views) before signal 'rowsRemoved'.

but the truth is: the rowsAboutToBeRemoved of TreeView's model scheduled something to delete delegate, but not direct call, so the delegate is deleted after signal rowsRemoved.

another solution is implement TreeView's attached signal ‘remove' like other views, then we can response it for release resources retained from model.

the Treeviews' behaviors about signal 'rowsAboutToBeRemoved' and 'rowsRemoved' is different with other views'.