closes#917
Summary follows:
- src/menu/menu.c: add `leave` action to call lab-exit.
- clients/meson.build: add install routine for lab-exit
and lab-sensible-terminal.
- meson.build remove lab-sensible-terminal routine re the comment.
- po/labwc.pot: rebuild with new xgettext commands.
- po/it.po: add translations for lab-exit and edit my email.
- CONTRIBUTING.md#translations: modify the instructions for coders as
two xgettext commands are needed since we now have two GUI facing
languages that have translatable strings.
