Patch from Kouhei Sutou (#6905).

	* glib/poppler-document.cc:
	* glib/poppler-document.h:
	* glib/poppler-private.h:
	* glib/reference/tmpl/poppler-private.sgml:
	* glib/reference/tmpl/poppler.sgml: Make PopplerPSOutput a proper
	glib object.