like in the webinterface: if the user hovers over the boundary of a shape, a point appears and by clicking and dragging you can add a new point to the shape and move it to the desired location
feature was already enabled but disabled again for unknown reasons...
Feature has been disabled because now you can move a boundary line by dragging it and this conflicts with this feature.
Thus, this feature should be enabled only when a user clicks ctrl + hovering over a boundary line.