Fixed: Memory leaks in CPOutlineView + corrupted itemInfos dictionary

Previously, when reloading a CPOutlineView with different datas than before, the CPOutlineView kept a reference of the previous datas.
Now, when reloading, the CPOutlineView will delete these references when we don't need it anymore. For that, the CPOutlineView will only try to delete the previous reference when we have reloaded the children of a node, otherwise it will keep the other references.

This PR fix some issue with the following methods :

- (BOOL)isExpandable:(id)anItem;
- (BOOL)isItemExpanded:(id)anItem;
- (CPInteger)rowForItem:(id)anItem;
- (CPInteger)levelForItem:(id)anItem;
- (id)itemAtRow:(CPInteger)aRow;
- (CPInteger)levelForItem:(id)anItem;
- (CPInteger)levelForRow:(CPInteger)aRow;

Previously these methods took care about not displayed items. Now they just work with displayed items.

Added unit-tests in Test/AppKit/CPOutlineViewTest.j

Test app in Test/Manual/CPOutlineViewTestCib