Implement a combobox where the entries are filtered by the line edit

dropdown_filter_combobox.cpp

        // Initialise the combobox for the edge selection
        // listmodel stores the actual data. This will be populated when we get a map update
        edgeIDModel = new QStringListModel();
        // Set the model for the combobox as the base stringlist model. If we use the same filter model for both the 
        // combobox and the completer, there will be weird behaviour, probably because the two objects are fighting
        // to define what is in the list of strings.
        actionComboBox->setModel(edgeIDModel);
        // Need a completer so that the combobox pops up with possible options given typed text
        edgeIDCompleter = new QCompleter();
        edgeIDCompleter->setParent(parent);
        edgeIDCompleter->setCompletionMode(QCompleter::PopupCompletion);
        // Make a filter model for the completer which allows sorting and filtering of data in the list model
        edgeIDFilter = new QSortFilterProxyModel();
        edgeIDFilter->setSourceModel(edgeIDModel);
        edgeIDFilter->setFilterCaseSensitivity(Qt::CaseSensitive);
        // the completer uses a different model to the combobox, but it points to the same base data (?)
        edgeIDCompleter->setModel(edgeIDFilter);
        // set the completer on the combobox so it will do the filtering
        actionComboBox->setCompleter(edgeIDCompleter);
        // Connect up the line edit to the filter so that when text is edited it modifies the
        // completer's model (and not the one of the combobox)
        connect(actionComboBox->lineEdit(), SIGNAL(textEdited(QString)), edgeIDFilter, SLOT(setFilterFixedString(QString)));