xgettext: Improve support for string formatting through methods.

xgettext: Improve support for string formatting through methods.

* gettext-tools/src/xg-arglist-context.h: Add comments.
(struct formatstring_region_ty): Add 'pass_format' field.
(struct flag_region_ty): Add subregion, nsubregions, nsubregions_max,
inherit_from_parent_region fields.
(new_sub_region): New declaration.
(set_format_flag_on_region): New declaration.
* gettext-tools/src/xg-arglist-context.c: Include xg-message.h.
(the_null_context_region): Update initializer.
(inheriting_region): Initialize the new fields.
(new_sub_region): New function.
(unref_region): Unreference also the subregions.
(set_format_flag_on_region): New function.
* gettext-tools/src/xg-message.h (set_format_flag_from_context): Fix typo in
comment.