π-cross: Prevent marking over a filled square

On other iterations of nonograms, they implement a priority system such that if you drag-fill a row, it won't overwrite existing marks (unless they're "tentative" marks). Similarly, if you try to re-mark a singular box, it instead deletes the marking on the first press, essentially building in a confirmation to prevent accidental errors. This would be a good QOL feature to add!