message: Fixed - impossible to customize panel class
diff: <template>
  - <div v-if="question.isVisible" :class="css.panel.container" :style="rootStyle">
  + <div v-if="question.isVisible" :class="question.cssClasses.panel.container" :style="rootStyle">
  <h4 v-show="hasTitle" :class="getTitleStyle()" v-on:click="changeExpanded">
  <survey-string :locString="question.locTitle"/>
  <span v-show="showIcon" :class="iconCss"></span>
  </h4>
  - <div v-show="hasDescription" :class="css.panel.description"><survey-string :locString="question.locDescription"/></div>
  + <div v-show="hasDescription" :class="question.cssClasses.panel.description"><survey-string :locString="question.locDescription"/></div>
  <survey-errors :question="question"/>
  <div :style="{ paddingLeft: getIndentSize(question, question.innerIndent) }" v-show="!isCollapsed">
  <div v-for="(row, index) in rows" :key="question.id + '_' + index" v-if="row.visible" :class="css.row">
  