Drag and Drop
Call add_drag_dest() to allow items or groups to be dragged from the tool palette to a particular destination widget. You can then use get_drag_item() to discover which ToolItem or ToolItemGroup is being dragged. You can use dynamic_cast to discover whether it is an item or a group. For instance, you might use this in your drag_data_received signal handler, to add a dropped item, or to show a suitable icon while dragging.
See the Drag and Drop chapter for general advice about Drag and Drop with gtkmm.