[collections] Initial addition of collection view model feature.
[collections] Rename pass #2.

Everything updated to use collection view.
[collections] More renaming and register cells.
[collections] Fleshing out size calculations.
[collections] Add support for item size calculation.
[collections] Provide access to the cell factory classes rather than embed size calculations.

Different layouts calculate sizes in different ways so there's no effective
way to define generic layout calculation for collection items.
[collections] Docs and delete the mutable collection view model because it's untested.