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.cpp

@@ -709,6 +709,11 @@ QDateTime timestampToDateTime(timestamp_t when)
 	return QDateTime::fromMSecsSinceEpoch(1000 * when, Qt::UTC);
 }
 
+timestamp_t dateTimeToTimestamp(const QDateTime &t)
+{
+	return t.toSecsSinceEpoch();
+}
+
 QString render_seconds_to_string(int seconds)
 {
 	if (seconds % 60 == 0)