Cleanup: use DiveTripModelBase::clear() to reset the log

Introduce a DiveTripModelBase::clear() function that cleanly
clears all dive data inside a beginResetModel()/endResetModel()
pair. Thus, the UI will be cleanly reset and we can remove
explicit calls to
 - graphics->setEmptyState()
 - mainTab->clearTabs()
 - mainTab->clearTabs()
 - diveList->reload()
from MainWindow::closeCurrentFile().

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

qt-models/divetripmodel.cpp

@@ -372,6 +372,13 @@ void DiveTripModelBase::resetModel(DiveTripModelBase::Layout layout)
 		currentModel.reset(new DiveTripModelList);
 }
 
+void DiveTripModelBase::clear()
+{
+	beginResetModel();
+	clear_dive_file_data();
+	endResetModel();
+}
+
 DiveTripModelBase::DiveTripModelBase(QObject *parent) : QAbstractItemModel(parent)
 {
 }