Expose trace targets

Is your feature request related to a problem? Please describe.
I would like to be able to know which features were involved in a drawing that has traced enabled.
So for example the trace source has multiple features and I draw a new polygon that "uses" (traces) parts of some of these features. Then I would like to know which ones were involved.

Describe the solution you'd like
There's multiple ways this could be done and I'm not quite sure what would be the best approach.
Currently all the trace state is private and the features that were selected as targets are not remembered. We could keep a history between a draw start and draw end and expose this through a public getter. Or just have it as part of the drawend event. Or just emit an event when the trace state changes and leave it up to the consumer of that event to track the history.

If you would agree on the usefulness of this feature and help me decide on the approach I could whip up a PR for this.

Start by locating the drawing interaction's private trace state and the draw start, draw end, and drawend event entry points. Compare the proposed history, public getter, drawend data, and trace-state event approaches. Done means consumers can reliably identify which trace-source features contributed to a traced drawing through a documented public API.