Add a clear button to the Timeline Events tab controls that clears only timeline data

In addition to the button and the top of the screen "Clear all", this clear button would clear only timeline events. We may need to take some consideration of what we do with flutter frames that are linked to timeline events that we have since cleared via this new button (perhaps mark them as stale or null out their timeline event information?). We may not even need to store the timeline events with a flutter frame anymore with the new perfetto integration, so we should look into removing this or splitting the FlutterFrame class into legacy and current diversions.

从 Timeline Events 选项卡控件开始，将请求的按钮与现有的顶层“Clear all”行为进行比较。跟踪 timeline 事件如何与 FlutterFrame 对象一起存储，以及 Perfetto 集成如何使用它们。完成要求包括提供仅清除 timeline 的操作，并明确决定如何处理关联的 FlutterFrame 数据，包括是否应将旧版表示与当前表示分开。

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