Implement unique prefix on config values

[aussig]

Currently all config values are prefixed with X with isn't particularly unique or identifiable.

EDMC docs recommend that all plug-in config values are prefixed with a unique identifier, so implement a one-off migration to store all config values prefixed with BGST_.

On load logic:

1. Check for existence of BGST_ value first.
2. If present, use it. If not present, look for equivalent X value.
3. If X value present, use it and delete it. If not present, use default value.

On save logic:

1. Save to new BGST_ value.

Config entries to handle:

• [ ] XShowZeroActivity
• [ ] XAbbreviate
• [ ] XSecondaryInf
• [ ] XDiscordUsername
• [ ] XEnableOverlay
• [ ] XDiscordWebhook
• [ ] XDiscordTWWebhook
• [ ] XCurrentSystemID
• [ ] XStationFaction
• [ ] XStationType