vala: Interpret string literals lazily

vala: Interpret string literals lazily

* x-vala.c (P7_EOF, P7_STRING_END, P7_QUOTES, P7_QUOTE, P7_NEWLINE)
(UNICODE, IS_UNICODE, UNICODE_VALUE): Remove.
(phase7_getc): Remove.
(phase7_ungetc): Remove.
(phase3_get): Use 'phase2_get' directly to extract string
literals; use 'arglist_parser_remember_literal' instead of
'arglist_parser_remember'.
(literalstring_c): Declare external variable.
(extract_balanced): Remove the
'xgettext_current_source_encoding' setting to prevent encoding
conversion around 'arglist_parser_done'.
(token_ty): New field 'escape'.
* x-vala.h (SCANNERS_VALA): Register 'literalstring_c' as a
literalstring_parser.