Create a new core-layout layout

Create a new layout which provides the toolkit grid mixins as classes.
It also ensures the wrapper is the correct width. This can then be used
by new applications which want to use the new grid styles without having
to redefine the grid classes before the start.
Is it assumed breadcrumbs will be applied via components (once there is a breadcrumb component?)

It'd be good to add a more detailed comment here explaining that.