Extend the representative-item logic to handle ingredients specified
as the intersection of multiple groups. Also add mangling of item
button content, because comma for a multi-group ingredient is getting
formspec-escaped and then not de-escaped.
Commit 043f6081452365daaa033c58e0738527ccb64c3d broke a couple of things
in the course of its refactoring. This patch restores the "G" flag that
signals group ingredients, the preference for "default:" items as group
representatives (where a representative isn't specifically registered),
and the built-in registration of default:cobble as representative for
group:stone (because it's the most commonly-used item in that group).
