Remove watcher and maybe add example for refs usage

[langehm]

          This could be done without a watcher using [vue refs](https://vuejs.org/guide/essentials/template-refs).

We should minimize the amount of watcher we introduce with components!

Originally posted by @langehm in #345 (comment)

[langehm]

There is also sth wrong with the formating in the mucfiledropzone component - see in picture:

[langehm]

Usage could look like this:

<template>
  <muc-button @click="clearWarnings">Reset </muc-button>
  <muc-file-dropzone ref="dropZone" button-text="Dokument hochladen" max-file-size="1" maxFileSizeWarning="Eine Datei hat mehr als 2 MB und kann nicht angefügt werden." />
</template>
<script>
import { useTemplateRef } from 'vue';

const dropZone = useTemplateRef('dropZone');

const clearWarnings = () => dropZone.value.clearWarnings();
</script