GHC left some of the unused imports as `import X ()`

This is achieved by using `stack build --ghc-options=-ddump-minimal-imports`
and looking for the *.imports files.

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.