cleanup: create common QDateTime -> timestamp conversion function

In analogy to the timestamp -> QDateTime conversion, create a
common function.
1) For symmetry with the opposite conversion.
2) To remove numerous inconsistencies.
3) To remove use of the deprecated QDateTime::toTime_t() function.

Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>

core/qthelper.h

@@ -63,6 +63,7 @@ QString getPrintingTemplatePathUser();
 QString getPrintingTemplatePathBundle();
 int gettimezoneoffset(timestamp_t when = 0);
 QDateTime timestampToDateTime(timestamp_t when);
+timestamp_t dateTimeToTimestamp(const QDateTime &t);
 int parseDurationToSeconds(const QString &text);
 int parseLengthToMm(const QString &text);
 int parseTemperatureToMkelvin(const QString &text);