Covers and diglots (work-around)

In a diglot publication, there can be three versions of each periph, (1) having no side specified, (2) having content for L, and (3) having content for R. These are important e.g. for book introductions. The content of the periph can of course include material from any language, as long as that is specified inside.

The cover-generating code currently expects that periphs that define the content of covers get generated without any side being specified. I.e. the fully specified cover should might look something like this:

\zglot|\*
\periph front cover |id="coverfront"
\zglot|L\*
\mt The tile in the main language
\zglot|R\*
\mt The title in the second language

At present, the front matter file starts with a slightly confused state, and without the initial zglot above, the periph will be saved as for R, and not be used.