Give feedback when no cover art can be found

Submitted by Joachim Noreiko

Description

At first glance the Cover Art plugin seemed to do nothing at all -- for about three songs, it just showed a big picture of a CD case. I didn't know what this was meant to be or what it was supposed to do (it was enabled by default).

Suggestion: put a tooltip for when no cover art is found, saying something like 'No cover art is available for this song'.

Version: 0.10.0