zOrder Support

[SquareLi]

Added zOrder support for entity.

[markknol]

I would call it just 'order', this has nothing to do with a z-axis right?

[SquareLi]

When two objects overlap, the object has higher z-order will be on the top.

[volkipp]

I think 'order' makes sense to me as well, because an entity doesn't necessarily even have a sprite attached to it.

[npretto]

I'd honestly prefer "zOrder" to just "order", if i wanted to display something on top of something else, i'd look for a "z-something" property first, order may look like something more generic. But I'm not sure, looking at the code it seems that the order in which entities get updates is the same as they get rendered right? That is actually just a more generic "order" then