Implement editing, removing, and persisting servers in DeltaClientGtk

[ninjadev64]

Description

This PR implements editing, removing, and persisting servers in the server list for DeltaClientGtk.

Type of change

Please delete options that are not relevant.

• [x] New feature (non-breaking change which adds functionality)

[ninjadev64]

I've implemented the requested changes, and persisting settings to a config file.

• ConfigManager.swift is almost identical to the implementation in Client - the only difference being the removal of 1 line which would display a modal error (DeltaClientApp in ClientGtk doesn't support this yet)
• StorageManager.swift is a more minimal implementation than in Client - it does not have the instance-level FileManager wrapper methods, and it does not have methods for backups, GPU captures, etc.
• ConfigError.swift is identical to its implementation in Client
• Config.swift is almost identical to its implementation in Client with the only difference being that it uses default values for its fields instead of having an initialiser method

[ninjadev64]

All done :+1: