[nemo-qml-plugin-email] Introduce alwaysOnMode property and model role.

alwaysOnMode property indicates that one or more accounts are in always
up to date mode, this can be used if applications want to show a global
status of last changes to accounts. alwaysOnMode model role returns true if
the account is in always up to date mode, false otherwise.
True I'll add in both places, you are right this is very minor, but less of these signals we send, less bindings evaluations will happen in QML side :)