Add support for Opus files if compiled against TagLib > 1.8

FEATURES:
 * Added support for files in Opus codec if Amarok is compiled against recent
   enough TagLib (post 1.8). patch by Martin Brodbeck.

FEATURE: 312905
FIXED-IN: 2.8
REVIEW: 108542
DIGEST: Amarok now supports Opus codec with recent TagLib