Reset signal hub in edit mode

[Johannes Deml]

See f9ff403

This behavior is useful when using the experimental play mode introduced in unity 2019.3.
It makes sure no old signals are used in a newly entered play mode. This way it is not mandatory (even though I would recommend it) to unsubscribe from all signals since they are regenerated every time.

If you want to disable this functionality (for example if you use signals not on in play mode but also in the editor itself) you can do so by calling EditorPrefs.SetBool("ResetSignalsInEditmode", false);