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Sets the full interface between the DrawSketchHandler and a tool widget available somewhere in the UI.
Signalling is as follows:
1. On activation of the tool, DrawSketchHandler notifies the tool widget.
2. The tool widget retrieves tool information necessary for representation (type of widget, visibility, ...)
3. When the tool widget has created an appropriate widget, it notifies back a widget change.
4. The tool receives the widget handler and can now configure and interact with the widget.
