DataGridNode should generic support subtitles in cells. TimelineDataGridNode creates subtitles for CallFrame cells, but it should be possible to add an arbitrary subtitle. As part of the Timelines UI redesign, many grids will need to show subtitles which were previously shown in sidebar tree elements.
<rdar://problem/24444493>
This is mostly handled in TimelineDataGridNode, and doesn't warrant the extra work right now.