[improvement] Dialog: centered in screen (#1782)

example/pages/dialog/index.wxml

@@ -46,6 +46,7 @@
   </van-button>
   <van-dialog
     use-slot
+    title="标题"
     show="{{ show }}"
     show-cancel-button
     bind:close="onClose"

packages/dialog/README.md

@@ -84,6 +84,7 @@ Dialog.confirm({
 ```html
 <van-dialog
   use-slot
+  title="标题"
   show="{{ show }}"
   show-cancel-button
   confirm-button-open-type="getUserInfo"

packages/dialog/index.less

@@ -1,6 +1,7 @@
 @import '../common/style/var.less';
 
 .van-dialog {
+  top: 45% !important;
   width: 85%;
   overflow: hidden;
   font-size: 16px;
@@ -10,6 +11,7 @@
   &__header {
     padding-top: 25px;
     font-weight: 500;
+    line-height: 24px;
     text-align: center;
 
     &--isolated {
@@ -22,7 +24,7 @@
     padding: 25px;
     overflow-y: auto;
     font-size: 14px;
-    line-height: 1.5;
+    line-height: 20px;
     text-align: center;
     -webkit-overflow-scrolling: touch;