Support asynchronous action handlers

[martin-fleck-at]

At the moment, action handlers only support synchronous handling of actions:

export interface IActionHandler {
    handle(action: Action): ICommand | Action | void
}

However, since we already have an asynchronous nature with the server/client architecture, it would be great to add support for asynchronous action handlers too. They are (mainly) handled in the action dispatcher where dispatching of actions and execution of commands is already asynchronous anyways. Extending the capabilities of the action handler should therefore only be a minor addition to that functionality.

(originally triggered by #1605)