Add support for playing audio on slide view

[kou]

@nishidayuya implemented this in https://github.com/nishidayuya/rabbit-slide-nishidayuya-2025-04-18-tokium_mybest_drinkup_at_rubykaigi2025-play_sound_on_rabbit .

We can play an audio with GTK 4 something like the following:

gio_file = Gio::File.open(path: filename)
media_file = Gtk::MediaFile.new(gio_file)
media_file.play

If we can provide an API to play an audio to themes like Rabbit::Canvas#draw_* API, users can do it by their theme.

[kou]

Let's add a theme named slide-audio or something. Audio file can be specified as a slide property.