Add semantic validation to configurations.

[mvastarelli]

The import/export functionality of the application only performs minimal schema validation for json and no semantic validation. For example, it is possible to modify the configuration so that a coin uses a non-existent wallet, or that two wallets share the same id, or mining time is a negative number.

We should update the schema to be more stringent about data validation and create a set of "rules" to make sure that the data makes semantic sense as well.