Allow montage to flow as columns (packed) or rows (not packed)

[pliablepixels]

This feature will allow the montage layout to either flow as columns or row

as columns:
1 4
2 5
3

As rows:
1 2 3
4 5

The advantage of columns is that the images can be packed together with no space between them. When arranged as rows, the row will have the height of the 'tallest' monitor screen