* 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

This patch disables accidental triggering of double clicking when you
are just single clicking on multiple objects fast.

A bit scary when you get prompted to remove a file but you wanted to
select a file.

I also made sure all strings are correctly ordered.

* Add an option to print hovered files to a FIFO

This adds an env variable, `NNN_FIFO`, that can be set to a path that
`nnn` will open/create as a FIFO, and where every hovered file's path is
printed. This allows creating external perview/quick open plugins, ...

It can be compiled out with the make variable `O_NOFIFO`.

* Check filename ptr instead of full path (for FIFO)

* Add documentation to use NNN_FIFO in plugins

* Fix path sent to FIFO in empty dirs

The current code will start editor if the mime doesn't start with
"text/". But, we want the opposite.

Simplify the check by using `strncmp` instead.

We may improve by writing a helper: `start_withs` in the future.

This change also cleans -Wstrict-aliasing on Ubuntu 16.04 LTS

This shows the "n³" logo as  icon in application menus, when desktop file is
installed (using `install-desktop` make target).

"n³" on a black knight chess piece/black horse head.

Support `$_Z_DATA` environment variable per z's documentation dcd5541e7d/z.1 (L78).
`$_Z_DATA` allows changing the default location of the z database file.