Currently NodeHelper::fromAsString() can only return the sender only if the passed KMime::Content is directly part of the original KMime::Message node tree. This patch makes NodeHelper::fromAsString() non-static, so that it can also perform lookups for extra KMime::Content parts.
Typical usecase is to look up sender from a signed part encapsulated inside an originally encrypted KMime::Content. The signed part is not part of the original KMime::Message node tree, thus NodeHelper::fromAsString() would return an empty string. With this patch we can lookup the original KMime::Content that this part is attached to (in this case it would be the original encrypted part) for which we can perform the sender lookup.