Fixed incorrect 'parent' property re-declaration.

This caused the following warning in Xcode: "Property attribute in class extension does not match the primary class".
Introduced with following commit:
https://github.com/Augustyniak/RATreeView/commit/caf2abccebb71abe2fe73036803b9be1e9159140#diff-ed4da06bdf44c605bf7fabf6161822b3L31