signals, signalTracker: Add Signals.DestroyableEventEmitter

Another prerequisite of !2330, that shares some commits with !2541 (but keeping them separated for reviewing reasons).

The main point here was to be able to expose a pure-js object of type Signals.EventEmitter as a destroyable-type to be used in signalTracker, currently introduced as Signals.DestroyableEventEmitter (naming can be improved). This allows to basically have auto-disconnection when a such object emits a destroy signal as we do from some GObject's.

Now, In order to implement this properly sadly we need more stuff than expected:

• We must have connect_after on signal-emitter.
  • In any case we should decide what to do with it, because without AFTER support, it would be quite unsafe to use.
• Some circular dependencies make things a bit harder than it should be, so for example the pure JS implementation of TransientSignalHolder should probably just be moved to signals module to make it cleaner.