libtorc-core: Make TorcPlayerInterface the HTTP service (not TorcPlayer)

- not entirely sure how TorcPlayer originally was made the service. The
player should be a state machine and the interface should be the...
interface.

- not a simple change though. TorcQMLMediaElement is already a QObject
subclass, so make TorcHTTPService a plain object again and ensure each
TorcHTTPService implementation has a SubscriberDeleted slot for clean
subscriber removal.