fix: 修复Modal样式

[RyanLiu0235]

First of all, thank you for your contribution! :-)

Please makes sure that these checkboxes are checked before submitting your PR, thank you!

• Rebase before creating a PR to keep commit history clear.
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Extra checklist:

if isNewFeature :

• Update API docs for the component.
• Update/Add demo to demonstrate new feature.
• Update TypeScript definition for the component.

[RyanLiu0235]

1. Modal的展示与否既然是靠样式的display: none;而非a:if来确定，就要让这个操作是最高层级的，否则如果外面有对view设置额外样式，就可能导致Modal无法隐藏。
2. Modal目前没有自动在屏幕水平垂直居中，我实在想不出有什么场景需要这样处理，姑且认为是一个bug吧。