Outside production lists, syntax "`foo:bar`" already makes it possible to
refer to the production "bar" of group "foo". This commit offers the same
feature inside production lists. Similarly to the reference syntax,
prefixing with a tilde prevents the group from being displayed.

This commit also makes it possible to use "`:bar`" to refer to production
"bar" from a production list without a group name. This is especially
useful when one has a main (unnamed) grammar and one or several named
extensions that need to refer to it.