Support icons for custom EditingAction items in text selection menus

[7enChan]

Describe the bug

Summary

Readium currently lets apps add custom text selection actions with EditingAction(title:action:).

However, custom actions are backed by UIMenuItem(title:action:), which does not provide an image API. This means custom actions cannot show icons in the expanded iOS edit menu, while system actions or actions built with modern UIMenuElement / UICommand can show icons.

Current behavior

Custom EditingAction items appear without icons in the expanded system edit menu.

Expected behavior

Readium should provide a way for apps to optionally attach an icon to custom editing actions, so UIKit can display that icon in the expanded edit menu when appropriate.

How to reproduce?

1. In the TestApp or an integrating app, add a custom EPUB editing action using EditingAction(title:action:).
2. Select text in an EPUB.
3. Open the expanded iOS edit menu.
4. The custom action appears without an icon.

Readium version

3.8.0

OS version

iOS 26.4

Testing device

iPhone 16

Environment

Additional context

Use case

Apps may want to provide custom EPUB text selection actions such as Highlight, Add note, Translate, or Share, and have them appear visually consistent with native iOS edit menu actions.

For example, an app might want to express:

EditingAction(
    title: "Highlight",
    image: UIImage(systemName: "highlighter"),
    action: #selector(highlightSelection)
)

[stevenzeck]

#705 does some of the work. Icons in menu items via UICommand is only available in iOS 16+.

[mickael-menu]

PR #705 is outdated and likely won't be merged, but modernizing the editing actions remains planned. I don't have a specific timeline, so contributions are welcome if anyone wants to take this on before.