Add ability to customize matrix dropdown columns list view

[andrewtelnov]

Developers should be able to add/remove/reoder columns in the dropdown columns editor.

[andrewtelnov]

By default there are 4 columns: isRequired, cellType, name and title. Here is the code that removed isRequired and title and add "readOnly";

        SurveyEditor.SurveyQuestionEditorDefinition.definition.matrixdropdowncolumn.properties = [
            "cellType",
            "name",
            "readOnly"
        ];