[grid] Started refactoring grid into proper layout

It became clear when implementing more complicated interaction that
the layout refactoring I had planned needed to be done sooner than
later.

This is the first chunk of work to abstract layout into adapter
classes, and grid is the first such adapter. I am leaving list
support in-place confined in the view for now until layout is
finished so there are no list regressions. Therefore I have
sprinkled many FIXME's about layout and what should be obsolete
in the future.

Eventually list support will become a layout adapter and column,
header, and renderers will move into the layout. It's possible that
the column controller will be made obsolete entirely.

So what is all of this layout stuff? The list view was always fairly
simple, and layout (how things are positioned on screen) was computed
on the go during the rendering phase. This presents a major problem
though with more complicated layouts since layout essentially has
to be computed on render and any sort of interaction, though it was
never a big deal with list.

So now the layout is computed and cached when things change:

  - the view allocation changes
  - the view is scrolled
  - the model size changes
  - hamburgers

This allows the rendering and interaction phases to simply iterate
the layout's children to render children on screen or interact with
them. Life is easier and cleaner.

So rendering is now mostly complete for grid, there are a few
glitches. Interaction doesn't work at all anymore, but it'll come
next.

I was thinking about the epic burger I had last night:

  - 1/2 lb burger patty
  - 1 grilled chicken filet
  - layer of fried mac and cheese
  - layer of bacon
  - layer of swiss cheese
  - buffalo sauce
  - mayo
  - grilled onions

It was life changing. Or maybe validating. Or something. But the big
problem with this burger was the mess, and on top of that, I really
wanted two of these beasts. So what I am going to do in the future
is make a casserole of these burgers. Top the dish off with cheese
and maybe some cracker crumbles or something and bake shortly. Yes.