Localization

[dplochcoder]

Implement l10n through a variant type that can accept both raw strings and LocalisedString keys from TC.Localisation.

• The change from 's' to 'z' is intentional to help differentiate the types.
• This change is mostly source-compatible but not binary compatible. Rebuilds will be necessary for some mods which implement custom menus, code changes may be necessary in some cases.

This PR is built on top of #31.