Better fallbacks

This adds another fallback option for layouts that include alternatives, like "no+aliens". If "no+aliens" layout is not present, Squeekboard will try "no".

This should resolve misunderstandings like #93 (comment 131268)

Or perhaps not - we give no indication when we fall back. The user in question may have understood that the desired layout was supported but not exposed.

Anyway, this will at least make fallbacks work.