Added QSOsView.ClearDatesTimes method to clear the start and end dates
and times. This method is called when importing a new ADIF file to
prevent a warning message about end date and time being earlier than
the start date and time when importing an ADIF file whose earliest
QSO is later than the latest QSO in the ADIF file that was already
loaded in the program.
This completes issue #124.