When the craft guide is showing a craft, the output slot is now a button,
which causes the craft guide to show ways in which that output can be
used. This mirrors the way input slots are buttons that show recipes
for the selected ingredient. Usages of an item can be iterated through
in the same way as recipes for the item. This incidentally offers some
ability to retrace one's steps through a crafting chain, without storing
actual history.
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 043f608145 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).
