Support for Kropki dots

[Salt-Factory]

Is your feature request related to a problem? Please describe.
I think there's currently no support for Kropki dots in the grid, i.e., black or white dots in between two cells. I've looked at the preview image and fiddled around in the editor, but am not able to find how to represent them.

This is also necessary for #41, as kropki is listed as one of the puzzles to support in the future.

Describe the solution you'd like
A way to enter kropki dots on cell borders. The correct shapes are already present (under Shape > Shape > first option > dot > xs, but they can only be placed in the centre of a cell.

[Salt-Factory]

I'm a bit of an idiot, I just found "kropki" in the "Constraints" drop-down. However, I'm not sure how to "Turn border: ON", as the interface instructs. I feel like I'm missing something here.

[Salt-Factory]

Never mind, haha. It's in settings. So sorry.