新增PathGroup机制，解决多个path多个refD设置比较困难的问题

[wusonw]

          @hanyuesoyeon 需要设置相对 path。

如果我图形比较复杂，设置的path节点比较多，那我需要定义很多的rect框作为参照物，这很不方便，而且图形比较复杂的情况下，我可能无法定位参照框的位置和大小，能不能引入一种pathgroup的机制

Originally posted by @NewByVector in #3398 (comment)

[x6-bot]

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