xgettext: Add support for generic XML files

xgettext: Add support for generic XML files

* gettext-tools/src/xlocator.h: New file.
* gettext-tools/src/xlocator.c: New file.
* gettext-tools/src/its.h: New file.
* gettext-tools/src/its.c: New file.
* gettext-tools/src/xgettext.c: Include "xlocator.h" and "its.h".
(its_locators): New variable.
(long_options): Add --its option.
(extract_from_xml_file): New file.
(main): Handle --its option.
(usage): Document --its option.
* gettext-tools/src/Makefile.am (noinst_HEADERS): Add xlocator.h and its.h.
(xgettext_CPPFLAGS): Add $(INCXML).
(xgettext_SOURCES): Add xlocator.c and its.c.