Clear text node references after sub-tree has been cleaned up.

Task-number: QTBUG-34181
Change-Id: I861e192cee2c683efee79e7404a5c9d70b60691f
Reviewed-by: Gunnar Sletta <gunnar.sletta@digia.com>
Merge pull request #63 from sletta/textedit-fix

[qtdeclarative] Clear text node references after sub-tree has been cleaned up.