Allow arbitrary nesting of sub-variations in PGN input

Allow arbitrary nesting of sub-variations in PGN input

The parsing of subvariations is moved out of the state-machine parsing,
to allow arbitrary nesting. In stead of the entire sub-variation being
parsed by lex as Comment, it now returns Open and Close for the
corresponding parentheses, which are then handled in a wrapper routine
for yylex(): Myylex(). Problem was that yy_text is supposed to return
the entire parsed text of the returned symbol, which thus has to be
collected. To make this possible, yylex has to return everything that it
parses, (even white space!), and a symbol "Nothing" was added to return
in parsing rules that before just ate away irrelevant text without
returning anything.