Add an xmobar indicator and daily reminder for overdue backups

backup.sh shows the days since the last simple_backup (orange from 7,
red from 14); backup-reminder.timer notifies daily once it is overdue.

.config/systemd/user/backup-reminder.service

@@ -0,0 +1,11 @@
+[Unit]
+Description=Notify when the last simple_backup is 14 days old or more
+StartLimitIntervalSec=2h
+StartLimitBurst=4
+
+[Service]
+Type=oneshot
+ExecStart=%h/.config/xmobar/backup.sh remind
+# no notification daemon yet (e.g. caught up right after login)? retry a few times
+Restart=on-failure
+RestartSec=5min

.config/systemd/user/backup-reminder.timer

@@ -0,0 +1,9 @@
+[Unit]
+Description=Check daily whether a backup is overdue
+
+[Timer]
+OnCalendar=*-*-* 10:00:00
+Persistent=true
+
+[Install]
+WantedBy=timers.target