* src/gui-file.c (fill_save_menu): If recently used saver has been
removed, use default saver, not the first one.

* src/file.c
(unregister_file_opener, unregister_file_opener_as_importer): Fixed
leaks.
(unregister_file_saver): Fixed stupid bug (and leaks).

* src/plugin-service.h (struct _PluginServiceFileSaver): Added
default_saver_priority field.

* src/plugin-service.c
(plugin_service_file_saver_read): Read "default_saver_priority"
attribute.
(plugin_service_file_saver_initialize): Register as default saver if
default_saver_priority is non-negative.