[nemo-qml-plugin-email] Improve message priority setting.

X-MSMail-Priority header was used to trick old instances of Outlook
to show correct priority for emails, but nowadays this causes spam filters to
potentially mark the emails as spam in some situations, new versions of Outlook
already recognize the standard headers correctly.

Ref: http://wiki.apache.org/spamassassin/Rules/MISSING_MIMEOLE