fix: stencil addGroup

[sunven]

Description

bugfix: #4007

Motivation and Context

分组信息通常由服务端返回，那么会先调用stencil.addGroup, 再调用stencil.load来完成分组的显示, addGroup方法未监听mousedown事件,导致节点无法拖拽。

examples/x6-example-features 中补充了示例

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Enhancement (changes that improvement of current feature or performance)
• Refactoring (changes that neither fixes a bug nor adds a feature)
• Test Case (changes that add missing tests or correct existing tests)
• Code style optimization (changes that do not affect the meaning of the code)
• Docs (changes that only update documentation)
• Chore (changes that don't modify src or test files)

Self Check before Merge

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[x6-bot]

👋 @sunven

💖 Thanks for opening this pull request! 💖

Please follow the contributing guidelines. And we use semantic commit messages to streamline the release process.

Examples of commit messages with semantic prefixes:

• fix: don't overwrite prevent_default if default wasn't prevented
• feat: add graph.scale() method
• docs: graph.getShortestPath is now available

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We get a lot of pull requests on this repo, so please be patient and we will get back to you as soon as we can.

[NewByVector]

抱歉，这么久才来 Review 代码，这里没有考虑 removeGroup 的时候清除事件监听，我重新提一个 PR。