avoid API change in BlockType class, re #2766

lib/models/Courseware/Block.php

lib/models/Courseware/BlockTypes/Audio.php

lib/models/Courseware/BlockTypes/BeforeAfter.php

lib/models/Courseware/BlockTypes/BlockType.php

lib/models/Courseware/BlockTypes/Canvas.php

lib/models/Courseware/BlockTypes/DialogCards.php

lib/models/Courseware/BlockTypes/Document.php

lib/models/Courseware/BlockTypes/Download.php

lib/models/Courseware/BlockTypes/Folder.php

lib/models/Courseware/BlockTypes/Gallery.php

lib/models/Courseware/BlockTypes/Headline.php

lib/models/Courseware/BlockTypes/ImageMap.php

lib/models/Courseware/BlockTypes/Text.php

lib/models/Courseware/BlockTypes/Video.php

lib/models/Courseware/Container.php

lib/models/Courseware/ContainerTypes/ContainerType.php

lib/models/Courseware/StructuralElement.php