updated plugin for DKIM Verifier

Simplified the plug-in.

The new plug-in version needs at least the 1.3.0 version of the DKIM Verifier.
This update is not time critical as the old plug-in version works also with the new DKIM Verifier version 1.3.0, thought there can occur some issues if the DKIM result is saved and both classic and conversation view is used.