feat: support Notification action buttons on Linux - #54516

Description of Change

Notification's actions option and action event were macOS and Windows only. On Linux the buttons were dropped without an error, even though libnotify and most desktop notification servers support them.

Each button entry in actions is now added to the notification when the server advertises the actions capability. Pressing one emits action with that entry's index and a selectionIndex of -1, as on macOS. Buttons go through the same callback as the default click, so the activation token is picked up first and an app can focus a window from the handler on Wayland. selection entries and buttons without text are skipped.

Tested against the fake notification daemon in spec/api-notification-dbus.spec.ts, and on GNOME Shell 50.4 (Wayland): the buttons render, pressing one emits action with the right index followed by close, and focusing a window from the handler raises it over another app's window.

Checklist

• I have built and tested this change
• I have filled out the PR description
• I have reviewed and verified the changes
• tests are changed or added
• relevant API documentation, tutorials, and examples are updated and follow the documentation style guide
• PR release notes describe the change in a way relevant to app developers, and are capitalized, punctuated, past tense, and no longer than 80 characters.

Release Notes

Notes: Added support for Notification action buttons on Linux.