Minesweeper: Make the SquareButtons non-checkable

We manage the checked state of these buttons manually in the code, and
we don't want the user to interfere with it, which would be possible if
we put them in checkable state.

Games/Minesweeper/Field.cpp

@@ -230,7 +230,6 @@ void Field::reset()
  square.label->set_icon(square.has_mine ? m_mine_bitmap : nullptr);
  if (!square.button) {
  square.button = new SquareButton(this);
- square.button->set_checkable(true);
  square.button->on_click = [this, &square](GButton&) {
  on_square_clicked(square);
  };