Add per-app bandwidth counting machinery

• The TunnelScaffolding gains a new get_bandwidth_counter method that lets scaffolding implementations provide a per-app bandwidth counter.
• This gets updated by the InteractiveCopier inside onion-tunnel when traffic is sent and received over the tunnel.
  • Note that it doesn't capture any Tor network-induced overheads, like cell framing etc. (although those are still captured in the global bandwidth counters, they aren't attributable per-app)
• The BandwidthCounter type was extended briefly to be able to support this usecase (instead of just being used for a global counter).
• A terrible Java implementation of this concept demonstrates that it works by awkwardly hacking on to the existing app list view.

Closes #39 (closed).