Wrote a new FullMessageCombiner class that combines the header and body
parts of a message into one part.

Made Changes to the ComposerAttachments class to use the
FullMessageCombiner.

Added Assertions in the ctor of the ImapMessageAttachmentItem and
FullMessageCombiner.

Removed the headerPartPtr() and bodyPartPtr() methods from the
ImapMessageAttachmentItem.

Made the FullMessageCombiner::data() returns the bodyPart and headerPart
data directly without caching.