gui: Always convert file names to UTF-8 for use in syntax.

gui: Always convert file names to UTF-8 for use in syntax.

Syntax as understood by the lexer is always in UTF-8, so file names
have to be in UTF-8 too.  (The PSPP code that opens files based on
strings from syntax is already using utf8_to_filename() to convert
them properly before opening.)

Before commit 9ade26c8349 "lexer: Reimplement for better
testability and internationalization", the encoding of syntax
files was not well-defined.  It was reasonable, then, to put file
names in generated syntax in the file name encoding.

Commit 9ade26c8349 changed the encoding of syntax so that it was
always in UTF-8.  This meant that file names in syntax had to be
converted back into the file name encoding before trying to open
the files, and I made that change (you can see, for example, the
call to utf8_to_filename in do_insert() in
src/language/utilities/include.c).  But I forgot that the GUI
needs to convert its file names into UTF-8 when it is generating
syntax, so this commit fixes that up.