undo: clear undo-stack when clearing dive data

When the dive data is cleared, all pointers in the undo-stack become stale.
Desktop explicitly called Command::clear() in that case, but mobile doesn't.
Thus, move the clear() call into DiveTripModelBase::clear()

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

qt-models/divetripmodel.cpp

@@ -467,6 +467,7 @@ void DiveTripModelBase::initSelection()
 
 void DiveTripModelBase::clear()
 {
+	Command::clear(); // If we clear the dive list, all undo-information becomes stalte.
 	beginResetModel();
 	clear_dive_file_data();
 	clearData();