Replacement for https://github.com/pop-os/cosmic-comp/pull/1687, that
works correctly with multiple outputs.
We don't want another window to show a focus indicator while a window is
being dragged, so keep the window in the focus stack. If a window is
being moved out of a stack, change the focus from the stack to the
window.
`refresh_focus_stack()` doesn't seem to be called here, but for good
measure, make sure that calling that function also won't remove a
`CosmicMapped` from the focus stack if it is currently part of a move
grab for the seat.
7fd033295f