QHttpMultipart convenience methods

Constructing and composing a QHttpMultipart contains some aspects that are possible candidates for automating, such as setting the headers manually for each included part.

This task is about figuring out the ways to reduce these in the common typical case, naturally leaving the current manual definition as an option. As a reference, when issuing a default multipart with CURL, one does not need to manually set the headers.

It is not known what and how to exactly do this at the time of this writing but needs to be explored as part of this task.