doc/media-api: Add Metadata property to MediaItem
doc/media-api: Add Metadata property to MediaItem

The implementation uses a single property to expose the item metadata
not multiple for every single metadata entry.

mapfau/bluez,ComputeCycles/bluez,ComputeCycles/bluez,silent-snowman/bluez,silent-snowman/bluez,silent-snowman/bluez,pstglia/external-bluetooth-bluez,pstglia/external-bluetooth-bluez,mapfau/bluez,pkarasev3/bluez,pstglia/external-bluetooth-bluez,pkarasev3/bluez,ComputeCycles/bluez,ComputeCycles/bluez,silent-snowman/bluez,pstglia/external-bluetooth-bluez,pkarasev3/bluez,mapfau/bluez,pkarasev3/bluez,mapfau/bluez