Introduce `Page.Container` component

Right now the `Page` component acts a flex container, allowing to render `Artboard`'s and `SymbolMaster`'s in either a column or a row.

I'd like however to have unlimited flexibility rendering Pages by composing `Flex` components. By having some sort of arbitrary `Flex` container, I could layout items on a `Page` in complex grids if necessary.

This would require us introducing a new Component that can be utilized to compute Yoga layout, but doesn't actually result in any backed rendering.

@mathieudutour - I know this goes against some of your comments in #291, but I'm not sure how else to achieve this.

**note:** it doesn't need to be called `Flex`.

Start by reading the existing Page component and the discussion in issue #291, then trace how Artboard and SymbolMaster children are laid out through Yoga. Define what the new container must expose for composing arbitrary layouts and verify that it affects layout without producing a rendered backing object.

Written by the indexing model from the issue text.