- Add pico-theme.yml with a theme's API version, theme-specific default Twig config, registering theme-specific custom meta headers and defaults for Pico's `theme_config` config
- Add new `onThemeLoading(&$theme)` and `onThemeLoaded($theme, $themeApiVersion, &$themeConfig)` events
- Enable Twig autoescaping by default
Instead of using `*.config.php` files, use `*.yml` files to configure Pico. YAML is much easier to understand, more user friendly and (at least a bit) more error-tolerant, but still very powerful. Don't break BC by letting `PicoDeprecated` still read `config/config.php`.
