ContentPackages are defined in mod.yaml and list Installers that support them, but then the Installers and their SourceActions knew nothing about ContentPackages.
Also added BeforeInstall and AfterInstall sections for SourceActions in the Installers.
- Add prefixes to all message keys to provide context
- Use messages with attributes for some UI elements (dropdowns, dialogs, checkboxes, menus)
- Rename some class fields for consistency with translation keys