Add support for Action<TArg> type listeners

[Edvinas01]

Currently to use a custom listener you have to use a listener component or implement the IScriptableEventListener<TArg> interface. In most cases using a component is enough, however it would also be handy to have the ability to add listeners of type Action, which would allow to supply regular methods.

One thing to keep in mind is that adding this functionality will harm the traceability (the list of listeners on asset).

TODOs:

• Add support for Action<TArg>
• Update documentation