BOutlineListView: fix ItemUnderAt and accept NULL as superitem

ItemUnderAt was returning items that were not under the superitem (this
was fixed for EachItemUnder in hrev52210).
Make a NULL superitem mean the parent of the topmost items. Despite not
being explicitly documented in the BeBook, that's how BeOS works, the
MenuWorld test app uses it and, well, it's handy.

Change-Id: I2551e8ce874a6238c5e5fb1eb742e68e62d3928a
Reviewed-on: https://review.haiku-os.org/c/haiku/+/7359
Reviewed-by: Adrien Destugues <[email protected]>

headers/os/interface/OutlineListView.h

@@ -139,6 +139,8 @@ private:
 									int32 level, int32* _superIndex = NULL);
 			int32				_FindPreviousVisibleIndex(int32 fullListIndex);
 
+			status_t			_ItemsUnderSetup(BListItem* superItem, int32& startIndex,
+									uint32& baseLevel) const;
 private:
 			BList				fFullList;