Multimedia plugins now have a unique JSON metadata 'Keys' property.

They previously had the multimedia services they provided as keys. This
was a problem when several plugins were available on the same platform,
providing the same multimedia service, but with different features or
capabilities. Since they had the same key, only the first plugin in the
directory was loaded.

Nevertheless, it was actually working until commit 732dcfe7 in qtbase,
as all plugins were loaded even when sharing the same key.

The services a multimedia plugin provides are now declared in the
'Services' property. To preserve compatibility with third-party plugins,
if that new property doesn't exist it falls back to the 'Keys' property.

Task-number: QTBUG-31868
Task-number: QTBUG-31476
Change-Id: Ic3ba32eeef21b69b922bd3d4feb111101559d132
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Lars Knoll <lars.knoll@digia.com>