libtorc-core: Simplify TorcHTTPService subscription handling

- TorcWebSocket doesn't actually need to know about subscriptions and
now that TorcHTTPService is a QObject in its own right, just listen for
the QObject destroyed signal to clean up stale subscriptions.
libtorc-core: Listen for deleted subscribers in TorcWebSocket
libtorc-core: Omit internal slot in TorcHTTPService
libtorc-core: Add service list property to TorcHTMLServicesHelp