With this approach, we only need to store cycle->dummy_node instead of
cycle->preview_anchor and cycle->preview_parent and we don't need to care
about annoying cases like when the previewed window has no siblings or
when a window tracked by cycle->preview_anchor is destroyed.
We were using the word "osd" to describe the window switcher, but it can
be used with on-screen display (OSD) disabled by
`<windowSwitcher><osd show="false">`. Let's use "cycle" instead to avoid
confusion.
