clients/: add lab-exit to display an exit window with labnag.

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.

clients/meson.build

@@ -54,3 +54,7 @@ executable(
 	install: true
 )
 
+clients = ['lab-sensible-terminal', 'lab-exit']
+foreach client : clients
+  install_data(client, install_dir: get_option('bindir'))
+endforeach