Description of the new feature/enhancement

If features are going to be added that rely on the user providing a pane id, e.g. focus-pane $id or swap-panes -s $sid -t $tid it would be helpful to have a command (perhaps identify-pane) that identifies what the ids of each pane are. This would require making the currently internal _id public, or choosing some other method of identifying each pane.

This might look like (briefly?) showing the id centered in each pane (excuse the paint mockup)

Proposed technical implementation details (optional)