xgettext: Desktop: Correct the line numbers in the output and in error messages.

xgettext: Desktop: Correct the line numbers in the output and in error messages.

* gettext-tools/src/read-desktop.c (struct token_ty): Add a 'lineno' field.
(desktop_lex): Correct the line numbers in error messages. For a token of type
token_type_pair, store the line number.
(desktop_parse): Pass the line number from the token to
desktop_reader_handle_pair.
* gettext-tools/tests/xgettext-desktop-1: Update expected output.
* gettext-tools/tests/xgettext-desktop-2: Likewise.
* NEWS: Mention the change.