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.