Construct event channel outside of Context

[link2xt]

This allows account manager to construct a single event channel and
inject it into all created contexts instead of aggregating events from
separate event emitters.

#skip-changelog this is a refactoring

[flub]

Much better indeed.

All this cloning of the Receivers is a bit on the scary side, but really that problem already exists because of get_emitter so nothing worse there. But maybe one day we'll actually ensure there can be only one emitter.

[link2xt]

All this cloning of the Receivers is a bit on the scary side, but really that problem already exists because of get_emitter so nothing worse there. But maybe one day we'll actually ensure there can be only one emitter.

This can be ensured by:

1. Making Events non-cloneable again.
2. Making it impossible to extract Recevier clone from Events. This means there will be no more dedicated EventEmitter, Events itself becomes the event emitter.
3. Passing Events by reference to Context::new() so it can clone sender from it.

This is possible to do in #3422. On C side user will create dc_event_emitter_t, then pass it to dc_context_new.