[improvement] avoid use cell selector (#2714)

packages/panel/index.js

@@ -22,11 +22,12 @@ export default sfc({
       <CellGroup class={bem()} {...context.data}>
         {slots.header || (
           <Cell
-            class={bem('header')}
             icon={props.icon}
             label={props.desc}
             title={props.title}
             value={props.status}
+            class={bem('header')}
+            value-class={bem('header-value')}
           />
         )}
         <div class={bem('content')}>{slots.default}</div>

packages/panel/index.less

@@ -3,10 +3,8 @@
 .van-panel {
   background: @white;
 
-  &__header {
-    .van-cell__value {
-      color: @red;
-    }
+  &__header-value {
+    color: @red;
   }
 
   &__footer {

packages/panel/test/__snapshots__/demo.spec.js.snap

@@ -8,7 +8,7 @@ exports[`renders demo correctly 1`] = `
         <div class="van-cell__title"><span>标题</span>
           <div class="van-cell__label">描述信息</div>
         </div>
-        <div class="van-cell__value"><span>状态</span></div>
+        <div class="van-cell__value van-panel__header-value"><span>状态</span></div>
       </div>
       <div class="van-panel__content">
         <div>内容</div>
@@ -21,7 +21,7 @@ exports[`renders demo correctly 1`] = `
         <div class="van-cell__title"><span>标题</span>
           <div class="van-cell__label">描述信息</div>
         </div>
-        <div class="van-cell__value"><span>状态</span></div>
+        <div class="van-cell__value van-panel__header-value"><span>状态</span></div>
       </div>
       <div class="van-panel__content">
         <div>内容</div>