max-rows overflows on reorder

When max-rows is set, columns can have more than max-rows rows when items are dragged into said row.

For a specific example: I have an 8x4 layout with 4 rows and 8 columns. When I fill the first column with 4 rows, then I drag an item into the column in the middle ([0,2]), then the last item gets pushed down. When I finish the drag event, the first column now has 5 rows, which is more than max rows.

Proposed fixes:

1. prevent dragging into a full row.
2. When dragging into a full row, overflow the item out of the row. Place it in the first unused row in the first non-full column.