zOrdering bug with locked parent

[wouterlucas]

If you create a locked parent with child nodes, the child nodes never seem to be able to update their zIndex if created in a different sequence. Ie. if created in the correct order as the zIndex prescribes it works but changing the z Index of childs within a locked parent doesn't seem to work.

For example:

parentNode
 zIndex: 1
 zIndexLocked: 1
 
 childNodeOne
   zIndex: 40
   
 childNodeTwo
   zIndex: 2

In the above childNodeOne will not be above childNodeTwo, even though it has a higher zIndex.

Expectation is that childNodeOne will be on top of childNodeTwo, but never block something that has a higher zIndex then parentNode.

[wouterlucas]

Fixed by changing zOrdering strategy per #91