Resolve "No picture in embedded player"

Closes #1697 (closed)

This MR adds support for all cover sources in the embedded player. Currently, the embedded player only checks to see if the album associated with a track contains a cover, and returns the default cover if not. Many channels don't use albums or series (see this forum post). This MR proposes the following approach:

1. Look for covers in the following locations in this order:

• The track
• The album/series
• The artist

2. Return the first cover object for the embedded player. Return the default cover if none are found.

Before

After