This cleans just a third of the warnings in this metadata plug-in. It is
a very bad practice to define static const strings in a header file like
this, especially if this header is included in several files. Let's make
these macros.

Also why are these header tags sometimes used with the const name (now
macros), sometimes directly with the string literals? I fixed some of
these, but more string literals are lying around.

Remaining warnings are similar, about const string arrays. If really we
want this, we should declare these extern and define them in a separate
implementation file. This whole plug-in should be really reorganized and
cleaned in depth.