Add platform overrides to project localisation

Add optional platform overrides to project localisation so one stable string key can display different text on Windows, macOS and Linux.

Keep an Any platform translation for each locale and allow individual values to be overridden where a platform needs different wording or menu mnemonic markers. Examples include Quit versus Exit, Windows menu captions such as &Edit, and translated menu names whose wording differs between operating systems. Existing Localise(...) calls and menu localisation bindings should select the appropriate value automatically.

Keep the language comparison table and add an Any platform / Windows / macOS / Linux selector to the localisation editor. Show inherited values distinctly, with explicit actions to create an override or restore inheritance. Most translations should remain shared.

Preserve language priority during fallback: within each locale, try the platform override before its Any platform value, then continue through the existing locale fallback chain. A shared German translation should therefore win over an English platform-specific translation.