GStreamer: Remove tags when an invalid QVariant is assigned.

Clear the tag list before syncing with reference set so old values
are removed and explicitly remove invalid QVariants from the reference
set so that the type correction doesn't result in invalid values
being written instead.

Change-Id: I7d1bdc95cd2a2d601720db84c9b3ef629477bc99
Reviewed-by: Yoann Lopes <yoann.lopes@digia.com>