Decouple qmediametadata.h from qmultimedia.h

qmultimedia.h is included in more places, but qmediametadata.h is
larger. This patch should reduce unnecessary #include-ing.

Change-Id: I4a3d174bafc555d794bb75087c1f6b79745ae903
Reviewed-by: Yoann Lopes <yoann.lopes@digia.com>