refactor(sdk): implement `Network#displayName`

[dated]

Summary

There are some occurrences in the Desktop Wallet where the networks coin name is concatenated with the network name, e.g. `${network.coin()} ${network.name()}`. The DW also uses some hardcoded display names. For live networks the network name is generally omitted, hence the check in the displayName method.

Checklist

• [ ] Documentation (if necessary)
• [x] Tests (if necessary)
• [x] Ready to be merged