Base class for controls that appear between blocks in a sequence, to allow inserting new
blocks at that point. Subclasses should render an HTML structure with a single root element
(replacing the placeholder passed to the constructor) and set it as this.element.
When the user requests to insert a block, we call onRequestInsert passing the index number
and a dict of control-specific options.
Base class for controls that appear between blocks in a sequence, to allow inserting new blocks at that point. Subclasses should render an HTML structure with a single root element (replacing the placeholder passed to the constructor) and set it as this.element. When the user requests to insert a block, we call onRequestInsert passing the index number and a dict of control-specific options.