[nemo-qml-plugin-email] Modified message body to behave as proper QML pr...

...operty

Consider following type of scenario:

Item { 
    id: emailComposer
    property alias emailBody: message.body

    TextArea {
        id: textArea 
    }

    EmailMessage {
        id: message
    }

    Component.onCompleted: {
        textArea.text = message.body
    }
}

One would expect that anything initially stored to emailComposer.emailBody
will end up to textArea.text but that's not the case with current EmailMessage
implementation. Modified EmailMessage to actually report changes in "body"
property. Do you see any problems in this approach?