The former two provide fallbacks in case $PAGER or $EDITOR is not set (which
might be more common than you think, because they have to be set in
~/.xsession, not in the shell configuration!) while the latter tries to launch
a terminal emulator. The scripts are most prominently used in i3-nagbar, which
alerts the user when the configuration is broken for some reason. Also,
i3-sensible-terminal is used in the default configuration.
This commit does not rely on the shell supporting ${PAGER:-less} anymore, which
is not the case for 'fish'.
David Coppa