Used to group bookmarks together, can contain default data.
Key of the bookmark is used as localization tag as well.

bookmark_group_1 = {
	### Breif: default_start_date (date)
	# Bookmarks that are assigned this group will use this as their default start 
	# date, can be overwritten by the bookmark itself.
	default_start_date = 867.1.1
}
