Use Strategy pattern for Goals

[tcrasset]

We could have one different Class for each GoalType.

Also, a different class for all the kinds of MoneyTransactions, so that we could make the add/delete Transactions have less if else statements.

--> This is not the strategy pattern

[tcrasset]

This can be coupled with #36, which uses the Factory pattern (we can use Factory constructors in Dart).

[tcrasset]

Actually, the strategy pattern is not the thing that we want for transactions.