* src/file.h
(typedef enum FileProbeLevel) New.
(typedef GnumFileOpenerProbeFunc, gnum_file_opener_probe): Added
FileProbeLevel argument.
(GNUM_FILE_OPENER_CLASS, IS_GNUM_FILE_OPENER_CLASS,
GNUM_FILE_SAVER_CLASS, IS_GNUM_FILE_SAVER_CLASS):
Macros moved to file-priv.h.

* src/file-priv.h
(struct _GnumFileOpenerClass::probe):
(GNUM_FILE_OPENER_CLASS, IS_GNUM_FILE_OPENER_CLASS,
GNUM_FILE_SAVER_CLASS, IS_GNUM_FILE_SAVER_CLASS):
Macros moved from file.h.

* src/file.c
(gnum_file_opener_probe_real, gnum_file_opener_probe):
Added FileProbeLevel argument.

* src/workbook-view.c
(wb_view_open_custom): Iterate over FileProbeLevel values when probing
the file (passing current value to probe function).

* src/xml-io.c
(xml_probe): Added FileProbeLevel argument, testing only file name if
probe level == FILE_PROBE_FILE_NAME.

* src/plugin-service.c (gnum_plugin_file_opener_probe):
Use file_patterns only if probe level == FILE_PROBE_FILE_NAME, otherwise
call module's probe function.

* src/bonobo-io.c (gnumeric_bonobo_io_probe),
src/plugin-service.h (struct _PluginServiceFileOpener),
src/plugin-loader-module.c
(gnumeric_plugin_loader_module_func_file_probe,
struct ServiceLoaderDataFileOpener::module_func_file_probe):
Added FileProbeLevel argument.