Docbook reader produces a `Div` with `title` class for `<title>` element
within an “admonition” element. Markdown writer then turns this
into a fenced div with `title` class attribute. Since fenced divs
are block elements, their content is recognized as a paragraph
by the Markdown reader. This is an issue for Docbook writer because
it would produce an invalid DocBook document from such AST –
the `<title>` element can only contain “inline” elements.
Let’s handle this invalid special case separately by unwrapping
the paragraph before creating the `<title>` element.
* XML.toEntities: changed type to Text -> Text.
* Shared.tabFilter -- fixed so it strips out CRs as before.
* Modified writers to take Text.
* Updated tests, benchmarks, trypandoc.
[API change]
Closes#3731.
