[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.
So, if we had a message stored in an external drafts folder, then we would make a new copy of it to go to the Sent folder, and delete the original?  If sending fails, does the new message end up in the Outbox?