templater: use template context to render old-style list template
Prepares for dropping the 'templ' resource.
This means old-style list templates are processed by the same engine class
as the one for the list node. I think that's fine since templates for the
same list should be tightly coupled, and I believe the extension point for
the engine classes isn't actually used.
Now templatekw._showlist() is a compatibility wrapper for _showcompatlist(),
and will be deprecated soon. The function is still marked as private since
I plan to change the interface to get rid of closures capturing context and
mapping.
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<description><![CDATA[{desc|strip|escape|websub|addbreaks|nonempty}]]></description>
<author>{author|obfuscate}</author>
<pubDate>{date|rfc822date}</pubDate>
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