Plugin code cleanup (removed unnecessary function arguments) + small
fixes.

* src/func.[ch]
(struct _FunctionDefinition):
Removed get_full_info_callback_data field.
(function_add_name_only): Removed callback_data argument.
(function_def_get_full_info_if_needed, FunctionGetFullInfoCallback):
Now we just pass fn_def instead of name + callback_data.
(function_call_with_list):
Added missing function_def_get_full_info_if_needed() call.

* src/plugin-loader.[ch]
(gnumeric_plugin_loader_load_service,
gnumeric_plugin_loader_unload_service):
Return error (instead of crashing) if service is not supported.
(gnumeric_plugin_loader_is_loaded): New function.

* src/plugin-service.h
(struct _PluginServiceGeneral, struct _PluginServiceFileOpener,
struct _PluginServiceFileSaver, _PluginServicePluginLoader):
Removed unneeded plugin argument from every function.

* src/plugin-service.c
(plugin_service_function_group_get_full_info_callback): Get fn_def
arg instead of fn_name + callback_data.
(plugin_service_function_group_initialize): Store pointer to service
in fn_def's user_data.
Adjusted to plugin-service.h (removed plugin argument).

* src/plugin-loader-module.c
(gnumeric_plugin_loader_module_func_init,
gnumeric_plugin_loader_module_func_can_deactivate,
gnumeric_plugin_loader_module_func_cleanup,
gnumeric_plugin_loader_module_func_file_probe,
gnumeric_plugin_loader_module_func_file_open,
gnumeric_plugin_loader_module_func_file_save,
gnumeric_plugin_loader_module_func_get_full_function_info,
gnumeric_plugin_loader_module_func_get_loader_type):
Adjusted to plugin-service.h (removed plugin argument).

* src/plugin.[ch]
s/plugin_load_loader_if_needed/plugin_get_loader_if_needed
(plugin_info_get_extra_info_list): Get loader if it's not available.
(plugin_info_is_loaded, plugin_info_get_loader): New functions.