Re-add showing of DKIM result in the statusbarpanel

[lieser]

This was removed as part of the conversion to an MailExtension for TB 78.

Consider re-adding an this option, especially in case Thunderbird will add an API for this in the future.

A user suggested as an alternative to add the icon previously used in the status bar before the from address.

[lieser]

Where is still no API for this, and I don't expect Thunderbird to ever add one.
I would also be hesitant to add even further experiments for this feature, even if someone else contributes it.

So closing this as not planned. If in the future unexpectedly an API to extend the status bar will be added, this could be revisited.