Give Mine Layers a "Replace" mode

[Slobjok]

That mode will automatically remove an existing mine (regardless if enemy or own) and place a new one, when the mine layer enters a field. If the entered field has no mine, then no mine is placed. If the mode is activated and the mine layer is on a field without a mine, nothing happens. If the mode is activated and the mine layer is on a field with a mine, then the mine is replaced.