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.