SyncClient listeners

[vaind]

#136 brings Sync but without listeners for events. We should add those as well for completeness.

How would that work? Take a cue from observable.dart?

[Buggaboo]

I couldn't find objectbox.h 0.11 (yet), when I was looking for callbacks for sync, if those are even emitted from the backend, from a duplex connection? Long-polling?

[vaind]

the C API's haven't been released yet but they look very similar to obx_observe(), i.e. you give the C-API a callback and the C will execute it (or not) at some point in the future

[vaind]

Implemented by #145