Port deprecated QLayout::margin call

As the method impl suggests, the left value is preferred if all the values are the same.
This is the case for the given layout.

```cpp
int QLayout::margin() const
{
    int left, top, right, bottom;
    getContentsMargins(&left, &top, &right, &bottom);
    if (left == top && top == right && right == bottom) {
        return left;
    } else {
        return -1;
    }
}
```