Add example songs to composer

[cdauth]

Right now the song in compose mode is empty by default, which can be confusing for new users, as there is no indication what can be done there.

My suggestion would be to introduce a new section to the song picker dropdown that contains one example song per tune. The example songs from the default tunes object can be used. By default, the song of the Tune Of The Year would be selected. The example songs cannot be renamed or deleted, but they can be copied. Editing an example song would immediately create a copy of the song that would be stored in the state.