commands/move: don't flatten on move; reap empty former parent instead
commands/move: don't flatten on move; reap empty former parent instead

Some comparisons of current Sway versus i3 behavior:

1) T[T[T[app]]] + move left
* Sway: T[app]
* i3:   T[T[app]]

2) H[V[H[V[app]]]] + move left
* Sway: H[app]
* i3:   H[V[app]]

After this commit, Sway behavior matches i3. The intermediate states
are now:

    T[T[T[app]]] -> T[T[app T[]]] -> T[T[app]]
    H[V[H[V[app]]]] -> H[V[app H[V[]]]] -> H[V[app]]

1ace/sway,1ace/sway,1ace/sway