Automatically update deferred fields for headers that have been modified by the template

If a template modifies headers (such as subject or to) and inserts a matching header variable in a New Email, it needs to be clicked to update. It would be better if in SmartTemplates can update these deferred variables automatically before the user starts to edit the email. This will save some clicks and time.

The deferred fields can still be updated again by clicking them another time (in case the user decides to modify the field manually - e.g. edits the subject line or adds a recipient).

Example:

RealRaven2000 / SmartTemplates

Automatically update deferred fields for headers that have been modified by the template #226