Move some macros to sx

This is some refactoring to facilitate voting and commenting.
It's written on top of #90 , so the diffs will get a lot shorter once that is merged.
It has been turned into `sx-tab-frontpage`, and we can now have other types of tabs as well.

I though I had explained that here, but it seems I screwed a bit and wrote that on #90 instead.

I'll post a comment here as well for the record.