Bugfix: read Performer ID3 tag from TPE4, not TPE3 - #497

Fixes issue #468.

Why: mpd recently changed its tag definitions, causing a mismatch against ncmpcpp's.

• Previously, "Performer" could be either TPE3 or TPE4
• Now, TPE3 is assigned to "Conductor" instead (in compliance with the ID3 standard)

What: updates definitions to match mpd's, so ncmpcpp can read and write the "Performer" tag without issues.

Haven't tested.