The List should ask the DataSource to populate the section headers

[etiennesegonzac]

Currently it does it itself [1] which is kind of weird design-wise and forces a section template on users.

https://github.com/gaia-components/fast-list/blob/f3f54c05f4683e23b5356d049f8845c35393aa39/scheduled-list.js#L263-L267