[nemo-qml-plugin-email] Emit sync changed signal when there's no enabled accounts and accountsSync is requested.

EmailAgent::accountsSync(..) can be called when there's no accounts in device,
in this case the function should exit properly and let the client know that no
sync will be performed.
Merge branch 'master' into vdv-devel
[nemo-qml-plugin-email] Add sync status to syncronizing changed signal.
Merge pull request #32 from nemomobile/vdv-devel

Emit sync changed signal when there's no enabled accounts and accountsSync is requested.