* Make Observer to be able to observe multiple Observables.
* Fix Observer destructor cleanup.
Co-authored-by: Sacha Weatherstone <sachaw100@hotmail.com>
because the ESP32 implementation will be different from NRF52
to make this possible I needed to decouple knowlege about bluetooth from
our mesh service. Instead mesh service now uses the Obserable pattern
to let any interested consumer get notified of important mesh changes
(currently that is only bluetooth, but really we should do the same
thing for decoupling the GUI 'app' from the mesh service)
@girtsf would you mind reviewing my Observer changes? I haven't written
C++ code in a long time ;-)
