msgfilter: Factor out quoted string handling
For later use in xgettext, separate out the scanner part in
filter-quote.c into a separate file. See:
<https://savannah.gnu.org/bugs/?44098>.
* quote.h: New file split from filter-quote.c.
* filter-quote.c: Include "quote.h".
(convert_quote_callback): New function.
(convert_ascii_quote_to_unicode): Use scan_quoted from quote.h.
* Makefile.am (libgettextsrc_la_SOURCES): Add quote.h.
