More loose ends of the file copy and paste feature.

	* libnautilus-extensions/nautilus-directory-notify.h:
	Rename NautilusFileChangesQueuePositionSetting to
	NautilusFileChangesQueuePosition. Add a field so it can
	be used to set or remove a position. Rename
	nautilus_directory_schedule_position_setting to
	nautilus_directory_schedule_position_set.

	* libnautilus-extensions/nautilus-directory.c:
	(nautilus_directory_schedule_position_set): Change name and also
	make it handle the case where we want to remove a position rather
	than setting it.

	* libnautilus-extensions/nautilus-file-changes-queue.h:
	* libnautilus-extensions/nautilus-file-changes-queue.c:
	(nautilus_file_changes_queue_schedule_position_set): Name change.
	(nautilus_file_changes_queue_schedule_position_remove): New
	function used to remove a position.
	(position_set_list_free): Name changes, added the new remove
	flavor where needed.
	(nautilus_file_changes_consume_changes): Updated SET cas...