After the user feedback (polygon like handling, instead of lasso while clicked), I would propose we implement a base tool (for add and remove) that behaves (mouse interactions and keys) like the lasso tool of GIMP. Only difference I would make, is that i would make it a closed contour from the beginning (anything else does not make sense in our context).

Key features:

• Normal state is polygon tool (click some where generates the next control point and a straigt line to it)
• Move while mouse button pressed, free lasso paint as now (when mouse button is release new controll point is generated)
• If courser is on an existing control point, we switch to control point moving mode (pressing mouse button starts moving the point)
• Return, Space (or like other tools (tool change, slice change...) finishes segmentation
• ESC cancles it
• Backspace, removes everything up to the last control point.