[improvement] Dialog: centered in screen (#3905)

src/dialog/demo/index.vue

@@ -33,10 +33,7 @@
       </van-button>
     </demo-block>
 
-    <demo-block
-      v-if="!$attrs.weapp"
-      :title="$t('componentCall')"
-    >
+    <demo-block :title="$t('componentCall')">
       <van-button
         type="primary"
         @click="show = true"

src/dialog/index.less

@@ -2,7 +2,7 @@
 
 .van-dialog {
   position: fixed;
-  top: 50%;
+  top: 45%;
   left: 50%;
   width: @dialog-width;
   overflow: hidden;
@@ -16,6 +16,7 @@
   &__header {
     padding-top: @dialog-header-padding-top;
     font-weight: @dialog-header-font-weight;
+    line-height: @dialog-header-line-height;
     text-align: center;
 
     &--isolated {