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
If there is no max_bottom the height is 0 of course #duh
SlotContents Forwardable --> SimpleDelegator
Restor manual looks/remove large offset

* My guess is that the old manual implementation had a 120 high
  stack there to get the offset from the top right. This much was
  needed as all the elements in the top were not considered for
  the slot height (the bug this branch fixes).
* I adjusted the hight to less and look good. Should actually
  look the same since the header is 90 pixels high, and the offset
  used to be 120, now it's 30
Merge pull request #773 from shoes/grow-slots-with-relative-elements

Consider relatively positioned elements for slot height

* Changelog: bugfix