* libanjuta/anjuta-plugin.h, libanjuta/anjuta-shell.[c,h],
	libanjuta/plugins.[c,h], libanjuta/interfaces/Makefile.am,
	libanjuta/interfaces/ianjuta-editor.h,
	(added) libanjuta/interfaces/ianjuta-document-manager.[c,h],
	plugins/editor/anjuta-docman.c,
	plugins/editor/anjuta-document-manager.plugin.in,
	plugins/editor/plugin.c, plugins/sample1/plugin.c,
	src/anjuta-app.c: Implemented IAnjutaDocumentManager interface.
	Added supported interfaces for plugins. Tested plugin interfaces
	through sample plugin.