addVariant

Caveat: this now belongs to the same plugin and is not registered as
separate variants which means that sort order can differ.

Now that a variant function can return a list of variant functions from
within the plugin, we have to make sure to executed and register those
functions as well.

However, we need to make sure that this list is local for the variant
and not "globally" registered otherwise we keep add a dynamic function
to the global list which results in duplicate output becaus multiple
duplicate variants will be registered.