Slot editor giving semicircle-ended slot of user defined router width, with detection of closed loop cutouts

When hand-panelising, or creating cutouts of any kind, it is sensible to be aware that the router width is fairly large, typically 2mm, and the ends must be semicircular. This way, the slots defined will accurately reflect the finished product.

For accuracy, it would be nice if the slots grew using the cursor keys (shift + arrow keys), and moved en-masse with the cursor keys (CTRL+arrow keys), in addition to mouse control. Slot direction could be via a middle button pop-up.

If the user wants to create a complete cut-out, then pressing <END> closes the loop. The result will be (for example) a rectangle with rounded corners. Of course, the shape can be more complicated.