Without system cron: warning and a backup that moves on while the page is open (#13); progress of the database phase; Bin tab

- The admin page and backup:status warn when background jobs do not run on a
  system cron, or the cron stopped, with a link to how to set one up.
- Without a working system cron, the open admin page moves a running backup
  forward through /api/tick, a short step per poll.
- The database phase shows the tables dumped ("db: 47/183 tables"); a backup
  that has not moved for 15 minutes says it is waiting for background jobs.
- The trash of restore points is on a tab of its own, Bin.
- New screenshots; changelog and plan for 1.0.2.

README.md

@@ -42,7 +42,8 @@ Need help restoring a backup after a crash? Ensembia offers paid restore assista
   background job, so a backup only moves forward when that job runs. Choosing "Cron" under
   **Administration settings → Basic settings → Background jobs** is not enough on its own: it
   tells Nextcloud that the system will call it, so you also need a cron line that does. Without
-  it a backup stays at "running (db)", and so does one started with "Back up now".
+  it scheduled backups do not run; the admin page and `occ backup:status` warn about it, and a
+  backup started with "Back up now" only moves on while the NG Backup page stays open.
   `occ backup:run` in a terminal works without cron (it runs in the foreground).
   - Regular server: `crontab -u www-data -e` and add
     `*/5 * * * * php -f /var/www/nextcloud/cron.php` (your Nextcloud path).