It was only possible to add a point on a current line and move it.
It is now possible to insert a point everywhere, a line will be created between this point
and the last existing one.
The key combinaison is alt + left click.
The point is pushed before the first point or after the last one depending on previous selection.

Please, note that some window manager (like KDE) already define alt + left click event,
it must be disabled to use this feature.