Set title/artist based on the file path if no tag is present

Prior to this change, untagged files would all have title and artist show up as unknown, making them impossible to tell apart. With this change, if no title/artist are provided in a tag the file stem/full path (respectively) are used instead.

For an untagged file at /home/amberol/path/to/My Wonderful Music.mp3:

Before:

Unknown title
Unknown artist
Unknown album

After:

My Wonderful Music
/home/amberol/path/to/My Wonderful Music.mp3
Unknown album