[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.
This handler checks to ensure that we only signal a change when one has actually occurred, while onAccountsRemoved doesn't.  I doubt it's important, but I would expect the same handling in both handlers...