* Unified preview-tui and preview-kitty

* Remove preview-kitty

* No need to set $TERMINAL

* fix undeclared variable

* Integrated support for scope.sh

* Review fixes

* allow_remote_control must be enabled on kitty

* Documentation, kitty splits and images fallback

* fix restoring kitty layout

In the dash shell, when `exec < fifo` is interrupted by SIGCHLD, it exits.
So we replace it with `cat fifo |`.

Issue discussed in #614

Fixed typo in preview-kitty.

Thanks @Kabouik!

* Added GPG encrypt and decrypt plugins

* Made description more descriptive

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

* preview-tui: simplify, add generic fifo_pager()

I've commented new filetype checks for now, as we need to discuss which ones
should be included by default, keeping in mind that this is supposed to be a
minimal, adaptable plugin.

* preview-tui: preview with man, tar, unzip by default

We leave it undefined on Solaris/Illumos because no such options exist,
so only use it if we have it.

Note that this is not compatible with 4 contexts.

This supports using a power of 2 number of contexts like 2, 4 or 8.
Note that the number of contexts are not backward compatible. So if
user is having 8 contexts and plans to use 4, his sessions with > 1
contexts open may break.

We can have a make option to enable 8 contexts. 4 remains default.

This reverts commit 2232772db4.

Cleanly handle SIGTERM and SIGHUP

This allow using both NNN_OPTS and options with arguments from argv.