* Add a tabbed/xembed based file previewer plugin
This plugin is written in bash, because job control is not well
specified in POSIX sh (`jobs` can return anything).
We use `tabbed` [1] as a xembed [2] host, to have a single window
owning each previewer window.
Uses mpv, sxiv, zathura, and the nuke plugin.
[1]: http://tools.suckless.org/tabbed/
[2]: https://specifications.freedesktop.org/xembed-spec/xembed-spec-latest.html
* tabbed-preview: prevent focus steal with xdotool
* preview-tabbed: tabs->4 spaces
* preview-tabbed: add focus prevention timeout
