Fixed: The delegate method shouldSelectItem doesn't work in CPOutlineView

Previously the delegate method shouldSelectItem didn't work in CPOutlineView.
It didn't work because the checking method _delegateRespondsToSelectionIndexesForProposedSelection and _delegateRespondsToShouldSelectRow didn't check if the real delegate of the outlineView implemented the methods
Changed comment
Merge pull request #2006 from Dogild/outlineView-selectItem

Fixed: The delegate method shouldSelectItem doesn't work in CPOutlineView