v1.29.0

### Changes

- New option "custom stylesheet" in the Stylesheets section: CSS inserted as the last stylesheet before the page is read, applied to the page and its frames and saved with the page
- The option "remove images for alternative screen resolutions" now also decides whether the alternative images are downloaded. Checked, only the displayed image of an `<img>` or `<picture>` is requested, which makes saving faster. Unchecked, the `srcset` variants are downloaded and kept with their media queries and sizes, so the saved page stays responsive. The hidden option `blockAlternativeImages` is removed from the settings: it defaulted to on, so unchecking the visible option used to leave empty `<source>` elements in the saved page instead of the variants
- single-file-core is updated from 1.6.25 to 1.6.28, see https://github.com/gildas-lormeau/single-file-core/releases/tag/v1.6.27 and https://github.com/gildas-lormeau/single-file-core/releases/tag/v1.6.28
- The image quality option is documented in the Chinese help page

### Fixes

- A text resource lost every U+FEFF it held instead of only a leading byte-order mark, so a stylesheet using the character as a zero-width no-break space in `content` was altered on save
- Grouped duplicate images are written in resource order instead of the order their downloads finished, so that part of the saved page no longer varies between two saves