tweak to ensure we don't accidentally invalidate the layout twice
use correct terminology in the data class for preparing layout
add a method on the layout that allows invalidation
remove the label on the grid demo for performance reasons
add a slider on the grid layout demo showing layout invalidation
we should floor here, not ceil. caused more visible cells than needed
refactor and implement the handling of layout invalidation
Merge pull request #86 from jwilling/layout-invalidation

Add layout invalidation