(feature) add method-callback to signal 'message-action-handler'

[rr-it]

This enables custom action handlers to make a callback to the default action-handler.

<template>
    <chat-window
        @message-action-handler="messageActionHandler"
    >
</template>

<script>
export default {
  name: 'ChatContainer',
  components: {
    ChatWindow
  },
  data() {
    return {
      messageActions: [
        {
          name: 'replyMessageCustom',
          title: 'Reply'
        }
      ],
  },
  methods: {
    messageActionHandler({ action, message, roomId, methodCallback }) {
      if (action.name === 'replyMessageCustom') {
        /*
         * Do something here.
         */
        methodCallback({ action: {name: 'replyMessage'}, message })
      }
    }
  }
}
</script>

What kind of change does this PR introduce? (check at least one)

• Feature

Does this PR introduce a breaking change? (check one)

• No

The PR fulfills these requirements:

• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)
• All tests are passing (The one and only test 'RoomsList.spec.js' passed.)

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature (to avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it)

Additional information:
The idea to feed-back a function inside the emitter is based on this approach:
https://stackoverflow.com/questions/55316490/vue-best-practice-for-calling-a-method-in-a-child-component/70723343#70723343

[antoine92190]

Thanks for the PR! Could you also add the new callback in the README? (in the Events API section)