Merge DisplayMode and PlaybackMode into a new class called MusicMode.
Both of these datatypes represented similar things, and thus it's much
easier to make them the same datatype. Moreover, it makes the
forthcoming addition of the music selector much easier if the same
datatype was tied to the representation of music.
This commit also moves around things around the project to be slightly
more coherent.
Revert the song menu in the playback panel to being in the toolbar.
It was mostly inconsistent and cause a lot of truncation. I'll figure
out what I want to do when I implement liked songs.
