Calls parent() only ~two out of three times and remove some
function calls in the TreeModel::parent() implementation,
reducing the overall footprint of this bit from ~5 to ~1.5%
when searching something in the project tree (test case
was Creator-in-Creator, and searching for 'dddddddddd',
resulting in 8 complete scans of the tree.

Task-number: QTCREATORBUG-17956
Change-Id: I4e46ef0467dd2aea58a7c944e1a2ee5c01e6fbba
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>