NodeHelper::fromAsString: make it work with extra content nodes

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.

Differential Revision: https://phabricator.kde.org/D3387