* Allow launching plugins with Alt + plugin's key

* Fix Alt key in filter/prompts modes

* Fix handling Alt key in nextsel()

In filter mode: run the associated plugin.
In prompt mode: just throw out the Alt+key input.
In nextsel(): differentiate Alt+key and Esc

Pass "master" as first argument to the script.

Co-authored-by: Todd Yamakawa <todd.yamakawa@arm.com>

As reported by @toddyamakawa

This fix previews window staying open after nnn exits, because they
inherited open FIFO (in write mode) from parent nnn process, and never
close it.

Uses `NNN_FIFO`, minimal dependencies.

Co-authored-by: Todd Yamakawa <todd.yamakawa@arm.com>

Co-authored-by: Todd Yamakawa <todd.yamakawa@arm.com>

* 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