Add sub-goals to the objectives

[retrohacking]

My plan is to add sub-goals (like steps that contribute to the main goal). Any objective can have as many steps as desired and these steps don't contribute to the active goals count.
The functionalities to add should be:
-Adding the subgoals related to a macro-objective
-Give the possibility to print an objective and its sub-goals
-Delete the subgoals (they don't contribute to the dismiss count)
These sub-goals have to be managed through a table of the database.