【WIP】Feat/tooltip label width buffer

[zamhown]

dom tooltip 的多行文本模式，可能会导致一些异常换行。原因是 canvas 的 measureText 的结果和 dom 环境的文本换行阈值不一致（measureText 计算出的宽度比 dom 需要的宽度略小）。

考虑支持 extraWidth 属性作为 tooltip 标签宽度的 buffer，在统一布局方法计算出 tooltip 标签宽度后，加上 extraWidth 属性并作用最终的 dom 标签宽度上。这样可以避免异常换行。

extraWidth 属性的默认值设在主题中，用户不用手动设置。

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[xile611]

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