Remove unslotted elements and add star method with 4 args

Fixes #508

* biggest contribution is the removal of the unslotted elements on the app, they now have  a parent
* `AbsoluteDimension` now purely exists to create a Dimension without a parent... which is kind of weird (that allows us to use `Float` numbers for width/height without that resulting in it being used as relative dimensions of the parent)
* star now takes 2 to 5 arguments