Install with `make install-desktop`
It now appears in application menus, search, etc. in XDG compatible
desktop environments (like GNOME, KDE, ...).

This reverts commit dbcbaa583c.

Replaces the following environment variables:
- NNN_USE_EDITOR
- NNN_IDLE_TIMEOUT

* Add support for Haiku OS

* Adjust DISTFILES and dist rule in haiku

This reverts commit 278fcfdfc9.

- remove auto-stored session "@" on restore
- minor help update

This reverts commit cf388649b9.

- replace bools for internal non-persistent binary states with an uchar
- do not store path to session, derive from config path in tmp buffer
- program option to load session is not `-s`
- remove auto-stored session "@" on restore
- minor help update

* Improved completion support for files with spaces

Filenames with spaces, both generally and for session names, will now
complete as expected.

* Misc shell script improvements

See discussion on:
1cca9e4b72