Display warning in Orphaned Panel when hierarchyMode = reference

Related to XWIKI-12520

Options:
1) Have an "if" in the panel and not display the title/content when hierarchyMode = reference.
2) Display a warning when hierarchyMode = reference explaining that this concept doesn't exist anymore with Nested Pages and that the panel should be removed.

Option 1 is transparent and works if you set hierarchyMode = parentchild but has the drawback of keeping the panel configured when hierarchyMode = reference.

So option 2 is probably better.