Right now, Mosaic operate on the windows itself. But this means that
swapping windows can act very unintuitively. This wrapper changes
mosaci to work on window /positions/ rather than windows themselves,
so the window in position 1 will always be the same size, and when moved
to position 2, it will inherit that position's size.

There's still some buggy behavior, but it is in general much more
intuitive than it was before.

This is mostly an academic exercise, as there's probably not much reason
to put a hole in the layout, but I must admit that sometimes is
aesthetically pleasing to see a little more desktop wallpaper in some
cases.