Fix selection when two single-window frames

With two single-window frames, `next-window` was being called with
`all-frames` as nil, resulting in the current window being returned
instead of the non-selected window. This commit sets the `all-frames`
argument to 'visible' and modifies `aw-switch-to-window` to account for
the two-window case where `position` may not be a window in the current
frame.

The only handler this commit modifies is `aw-switch-to-window` because
the behavior for `aw-delete-window` and `aw-swap-window` with multiple
frames doesn't seem to be well specified at this point.