Consider relatively positioned elements for slot height

* fixes #765
* adjustments in SlotContents to have the full power of
  Arrays at our fignertips without extending it all the time..
  maybe better suited as a simple delegator though

I'll leave some in code comments. :) Result looks good:

![screenshot from 2014-06-25 16 21 36](https://cloud.githubusercontent.com/assets/606517/3386456/ac9c25c2-fc74-11e3-8bbc-8d92eaf71b74.png)

So this was the easiest solution however we might want to embed this information into current_position/last_position for performance reasons as this does not need another iteration over all elements and also already has some max logic in it. As contents_alignment is called a lot this might be necessary.

I didn't see a good place to fit it in though (all this logic is skipped for absolutely positioned elements at the moment)