Define UI states
We're winging it when it comes to defining the UI states of the various media playback controls.
We need a proper definition for every state:
- the initial state of the UI when no song is loaded
- the state of the UI when loading the first song
- the state of the UI when the current song is the only one in the queue
- the state of the UI when the current song is the first in the queue
- the state of the UI when the current song is the last in the queue
- the state of the UI when the current song is playing
- the state of the UI when the current song is paused