Implement <calloutlist> and children.

[jaredmorgs]

Implement callouts (i.e, calloutlist)

Tracked from the issues marked with an asterix in https://github.com/opendevise/docbookrx/blob/master/WORKLOG.adoc

[mojavelinux]

We need to implement both callout numbers and the callout list items that correspond to those numbers.

Sample input:

<screen>line <co xml:id="CO1-1"/></screen>
<calloutlist>
<callout arearefs="CO1-1">
<para>explain that line</para>
</callout>
</calloutlist>

Expected output:

----
line <1>
----
<1> explain that line

Actual output:

----
line
----