Summary:
Avoid allocating dpointer when using QByteArray::fromRawData() by
introducing unfoldHeader() and Header::from7BitString() overloads
that work on const char* + size_t pair - this actually works nicely
for Structured headers which internally operate on raw char* pointers
instead of QByteArrays, so we avoid constructing an additional temporary
QByteArray in this case.

Note that this is an ABI incompatible change, make sure you rebuild
everything properly otherwise you'll get some really weird crashes.

Reviewers: vkrause

Subscribers: #kde_pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D8290