* src/Makefile.am (GNUMERIC_BASE_SOURCES): Added error-info.[ch]

* error-info.[ch]: New files.

* src/plugin.[ch]: New plugin engine.

* src/dialogs/dialog-plugin-manager.c,
src/dialogs/plugin-manager.glade,
src/dialogs/plugin-manager.glade.h:
New plugin manager.

* src/main.c (gnumeric_main): Call plugins_shutdown() on shutdown.

* src/bonobo-io.c
(gnumeric_bonobo_write_workbook, gnumeric_bonobo_read_workbook,
gnumeric_bonobo_io_probe): Added "user_data" arg.
(gnumeric_bonobo_io_init): Saving saver/opener id.

* src/file-autoft.c (gnumeric_extra_autoformat_dirs): Used
gnumeric_config_get_string_list function.

* src/file.c
(file_format_register_open, file_format_register_save): Added
user_data parameter, return FileOpenerId/FileSaverId.
(file_format_unregister_open, file_format_unregister_save): Take
only FileOpenerId/FileSaverId as parameter.
(get_file_saver_by_id, get_file_opener_by_id): New functions.
(do_load_from): Pass user_data parameter to "probe" and "open"
functions.
(workbook_save_as, workbook_save): Pass user_data parameter to "save"
function.

* src/file.h
(FileFormatProbe, FileFormatOpen, FileFormatSave): Adder user_data
parameter.
(struct _FileOpener, struct _FileSaver): Added opener_id/saver_id
field.
(file_format_register_open, file_format_register_save):
Added user_data parameter, return FileOpenerId/FileSaverId.
(file_format_unregister_open, file_format_unregister_save):
Take only FileOpenerId/FileSaverId as parameter.

* src/gnumeric-util.[ch]
(gnumeric_error_info_dialog_show, static ctree_insert_error_info):
New functions displaying dialog with ErrorInfo contents.

* src/gui-file.c (gui_file_import): Pass user_data parameter to
"open" function.

* src/gutils.[ch]
New functions: gnumeric_config_get_string_list,
gnumeric_config_set_string_list, g_string_list_copy,
g_strsplit_to_list.
(g_create_list): Added missing va_end().

* src/stf.c (stf_read_workbook, stf_write_workbook): Added user_data
parameter.

* src/workbook.[ch] (workbook_set_saveinfo): Now takes file_saver_id
instead of save_fn.

* src/xml-io.c
(xml_probe, gnumeric_xml_read_workbook, gnumeric_xml_write_workbook):
Added user_data parameter.
(xml_init): Store xml_opener_id and xml_saver_id.
(gnumeric_xml_get_opener_id, gnumeric_xml_get_saver_id): New
functions.