[FIX] modal: Don't return on first z-index found (might be nested)

git-svn-id: file:///var/local/code.tiki.org/tikisvn/trunk@47335 b456876b-0849-0410-b77d-98878d47e9d5
[FIX] modal: andSelf was deprecated in favour of addBack (thought it was the other way round, thanks sylvieg)

git-svn-id: file:///var/local/code.tiki.org/tikisvn/trunk@47336 b456876b-0849-0410-b77d-98878d47e9d5