format-table: introduce TABLE_HEADER cell type

format-table: introduce TABLE_HEADER cell type

This rework the logic for handling the "header" cells a bit. Instead of
special casing the first row in regards to uppercasing/coloring let's
just intrduce a proper cell type TABLE_HEADER which is in most ways
identical to TABLE_STRING except that it defaults to uppercase output
and underlined coloring.

This is mostly refactoring, but I think it makes a ton of sense as it
makes the first row less special and you could in fact insert
TABLE_HEADER (and in fact TABLE_FIELD) cells wherever you like and
something sensible would happen (i.e. a string cell is displayed with
a specific formatting).