Cleaned up file i/o interface.

* src/Makefile.am: Added file-priv.h.

* src/file.[ch]:
Made GnumFileOpener and GnumFileSaver GtkObjects with virtual probe,
open and save methods.
New GnumFileOpener methods: get_id.
New GnumFileSaver methods: get_id, fix_file_name.
New functions: register_file_opener, register_file_opener_as_importer,
unregister_file_opener, unregister_file_opener_as_importer,
register_file_saver, register_file_saver_as_default,
unregister_file_saver, get_default_file_saver,
get_file_opener_by_id, get_file_saver_by_id,
get_file_savers, get_file_openers, get_file_importers.

* src/file-priv.h: New file with definitions of structs:
_GnumFileOpener, _GnumFileOpenerClass, _GnumFileSaver,
_GnumFileSaverClass.

* src/workbook.h
(struct _Workbook): Added file_saver_sig_id field.
s/FileSaver/GnumFileSaver/
(workbook_get_file_saver): New accessor function.

* src/workbook.c
(cb_saver_destroy_event, workbook_destroy, workbook_set_saveinfo):
When setting new file_saver, connect to its "destroy" signal and
set file_saver to NULL in signal handler. Disconnect when changing file
saver or destroying the workbook.
(workbook_get_file_saver): New accessor function.

* src/workbook-view.[ch]
(wb_view_set_attributev): Renamed to wb_view_set_attribute_list and
simplified.
(wb_view_save_as, wb_view_save, wb_view_open, wb_view_open_custom): New
methods (some code moved from file.c).

* src/gnumeric-sheet.c (gnumeric_sheet_filenames_dropped):
Use wb_view_open() instead of workbook_try_read()+file_finish_load().

* src/gui-file.c
(gui_file_import): Use get_file_importers(). Use
wb_view_open_custom() instead of file_opener_open() + manual creation
of wb_view, error checking, etc.
(is_saver_registered): Removed.
(saver_activate): Don't check if saver is really registered.
(file_saver_is_default_format): Use get_default_file_saver() instead
of strcmp(extension, "gnumeric") hack.
(do_save_as): Use gnum_file_saver_fix_file_name().
Adjusted everything to file.h, workbook.h, workbook-control.h.

* src/gui-file.h (gui_file_import): Signature change, now function
returns gboolean.

* src/gutils.[ch] (g_list_index_custom): New function.

* src/history.c (file_history_cmd), src/main.c (gnumeric_main),
src/workbook-control-gui.c (cb_file_open): Use wb_view_open()
instead of workbook_read().

* src/workbook-control-gui-priv.h (struct _WorkbookControlGUI):
s/FileSaver/GnumFileSaver/

* src/plugin-service
Created GnumPluginFileOpener and GnumPluginFileSaver classes (derived
from GnumFileOpener and GnumFileSaver respectively). Moved contents of
plugin_service_file_opener_probe_func,
plugin_service_file_opener_open_func,
plugin_service_file_saver_save_func into
gnum_plugin_file_opener_probe, gnum_plugin_file_opener_open,
gnum_plugin_file_saver_save methods (slightly modified).
(plugin_service_file_opener_read): Read "open" and "import" attributes
of <service> tag.
(plugin_service_file_opener_initialize,
plugin_service_file_opener_cleanup): Register/unregister created
file opener (GnumPluginFileOpener object) as normal opener and/or
importer depending on "open" and "import" XML attributes.
(plugin_service_file_opener_initialize,
plugin_service_file_opener_cleanup): Register/unregister created
file saver (GnumPluginFileSaver object).

* src/plugin-service.h
(struct _PluginServiceFileOpener):
Added can_open and can_import fields.
s/FileOpener/GnumFileOpener/
(struct _PluginServiceFileSaver):
s/FileSaver/GnumFileSaver/

* src/plugin-loader-module.c
(struct ServiceLoaderDataFileOpener, struct ServiceLoaderDataFileSaver),
gnumeric_plugin_loader_module_func_file_probe,
gnumeric_plugin_loader_module_func_file_open,
gnumeric_plugin_loader_module_func_file_save):
s/FileOpener/GnumFileOpener/
s/FileSaver/GnumFileSaver/

* src/stf.c
(stf_read_workbook, stf_write_workbook): Signature change
(don't return gboolean).
(stf_init): Adjusted to file i/o interface changes.

src/xml-io.[ch]
(gnumeric_xml_get_opener, xml_probe):
s/FileOpener/GnumFileOpener/
(gnumeric_xml_get_opener):
s/FileSaver/GnumFileSaver/
(gnumeric_xml_read_workbook, gnumeric_xml_write_workbook): Signature
change (don't return gboolean).
(xml_init): Adjusted to file i/o interface changes.