Addresses: #35

Changes:

• Added a new view of the playing song to the listen-menu transient including an svg-lib progress bar. The song and artist are fontified, and are grey when the song is paused.
• New lighter format option "%R" which shows the song's progress in a svg progress pie (now the default instead of "%r").
• Add progress bar to the listen-status buffer.
• Automatic refreshing of the listen-menu transient.
• Fixed an issue where listen--elapsed returns nil when called too early. Now returns 0 when playback-started-at is nil.