oc-mailfolder: Avoid setting seen flag on preloading message bodies

Fetching a body[text] property using IMAP makes IMAP server set seen flag.

This commit fetches the flag beforehand to restore the previous state
once the body has been fetched.
oc-mailfolder: Get seen flag directly from message in preloading

To avoid a loop and a server-call.

We are not using body.peek[text] directly because bodyContentPart message
is explicitly avoiding it.
Merge pull request #50 from Zentyal/ejhernandez/keep-seen-flag-on-preload-msg-bodies

oc-mailfolder: Avoid setting seen flag on preloading message bodies