Serial Bus No. 4: Add subscribing to properties over serial bus

[JensOgorek]

This allows to subscribe to a property over the serial bus.

• Add SerialBus.subscribe(id, "module.property") control frames (__SUBSCRIBE__:module.property), letting a node subscribe to a remote property without a proxy.
• Subscribed peers send back lightweight updates as module.property=value (in normal bus frames) and only when the value changes; values use the standard property serializer (bools, ints, floats,
  quoted strings).
• Allow dotted assignments to auto-create placeholder modules/properties so subscribers can store incoming module.property=value lines out of the box.

Bus syntax examples:

• Request: $$<this_id>:<peer_id>$$__SUBSCRIBE__:input1.level
• Update back: $$<peer_id>:<this_id>$$input1.level=0 (or true, "foo", etc.)