[dataDate]: only update timestamp where data was modified

[MatthiasArnold]

when data is modified, only the changed elements should get updated time stamps.

example dateSet (cf. #352):
the user ONLY changed the Date from 2014 to 2015. this should be reflected in the time stamps, where dataDate is updated only in the two date subelements.

<dateSet dataDate="2015-11-13T10:43:48.23+01:00">
    <display/>
    <notes/>
    <date dataDate="2015-11-13T10:43:48.23+01:00" pref="false" type="creation">
        <earliestDate>
            <date dataDate="2015-11-13T11:00:10.19+01:00" circa="false">2015</date>
        </earliestDate>
        <latestDate>
            <date dataDate="2015-11-13T11:00:10.19+01:00" circa="false">2015</date>
        </latestDate>
    </date>
</dateSet>