Add panel for wp_mail() uses

[crstauf]

See https://github.com/users/johnbillion/projects/8/views/3?pane=issue&itemId=21798645.

[crstauf]

I want to include a Headers column, but it seems the headers array isn't populated in any of the actions (despite being included). 😖

[crstauf]

After analyzing wp_mail() more closely, the following headers are removed prior to the actions: from, to, cc, bcc, reply-to; explains why it seemed the headers array entry was empty.

[crstauf]