Custom LSB & RSB for Zero Width Mark Attachment

[GeorgSeifert]

@Bendy

I think we discussed this fairly recently, wasn’t the idea to put a non-exporting #collapse anchor in there to tell Glyphs where to collapse the sidebearings?

I just realised that there is a special anchor that does something very similar: “*origin”. It is meant for components that are used as pieces to build glyphs (stems, serifs …). I added support of it in the component alignment code.