[nemo-qml-plugin-email] Improve drafts handling

Some services(e.g gmail, outlook) automatically create emails in the
send folder when using their smtp and if after sending the imap client
does not append a email to the same folder.
When sending a draft that exists in the server side,
if we copy the existent email to the send folder it will cause a duplicate
due to the automatic creation by the server, with this commit we always create
new emails from existent drafts and then delete the drafts after enqueing them for
sending.
Yes, its stays in local outbox is a new message after all. Copy the draft after sending cause the mentioned problem(duplication) and also we would need extra round trip to update the message in case user changes the draft. These problems can be reproduce with Qtmail example app as well.