Added support for plugin dependencies + some fixes.

* src/gutils.h (g_list_to_vector, g_vector_free_custom):
New macros. (used macros instead of functions to avoid casting and
pointer size problems - ANSI C doesn't guarantee that
sizeof (void *) == sizeof (something_else *).

* src/plugin-loader.c
(gnumeric_plugin_loader_load): Load required plugin dependencies
before the plugin itself, increment n_deps for every loaded
dependency (by calling plugin_dependencies_inc_dependants).
(gnumeric_plugin_loader_unload): New function.
(gnumeric_plugin_loader_load_service): Increment number of loaded
services if loaded succesfully.
(gnumeric_plugin_loader_unload_service): Decrement number of loaded
services if unloaded succesfully, unload plugin if it reaches zero.

* src/plugin-loader.h
(struct _GnumericPluginLoader): Added n_loaded_services.
(gnumeric_plugin_loader_unload): New function.

* src/plugin-service.c:
(plugin_service_file_opener_open_func,
plugin_service_file_saver_save_func):
Used gnumeric_io_error_occurred (and gnumeric_io_clear_error) when
checking for error.

* src/plugin.c:
(plugins_active_by_default): Array containing ids of "standard"
plugins. These will be active by default.
(struct _PluginDependency): New struct.
(struct _PluginInfo): Added dependencies_v, d_deps. Replaced
service_list with services_v (vector in place of list).
(plugin_info_get_config_prefix, plugin_dependency_get_plugin_info,
plugin_info_read_dependency_list, plugin_dependency_free,
plugin_inc_dependants, plugin_dec_dependants,
plugin_dependencies_inc_dependants,
plugin_dependencies_dec_dependants,
plugin_load_dependencies):
New functions.
(plugin_info_provides_loader_by_type_str, plugin_info_free):
Adjusted to service_list -> services_v change.
(plugin_info_read): Read (optional) dependencies, adjusted to
service_list -> services_v change.
(activate_plugin):
Activate required dependencies before the plugin itself.
Increment n_deps for every activated dependency (by calling
plugin_dependencies_inc_dependants).
Adjusted to service_list -> services_v change.
(deactivate_plugin):
Decrement n_deps for every activated dependency (by calling
plugin_dependencies_dec_dependants).
Adjusted to service_list -> services_v change.
(plugin_can_deactivate):
Return FALSE if any of n_deps > 0.
Adjusted to service_list -> services_v change.
(plugin_list_sort_by_dependency): Removed.
(plugin_db_activate_plugin_list): Removed sorting.
(plugin_db_init): Always activate new plugin if it's defined in
plugins_active_by_default array.

* src/plugin.h:
(PluginDependencyType): New enum.
(plugin_load_dependencies,
plugin_inc_dependants, plugin_dec_dependants,
plugin_dependencies_inc_dependants,
plugin_dependencies_dec_dependants,
plugin_info_get_config_prefix): New functions.