This makes the output from PGF.showExpr (and other Haskell code that uses
the Prelude.show function to show strings) parsable as GF source code in
more cases.
This is a workaround for the problem that GHC's implementation of the show
function uses numeric escapes for printable non-ASCII characters, e.g.
show "dålig" = "d\229lig"...