set apart host backups of other nodes and of former jobs

- Each host backup carries the node it belongs to, read from its source host or its name (`hostcfg-<node>`, a job of this node or its PBS backup ID); both backup lists also return the node name.
- The backup list offers This node / All nodes when some backup belongs to another node, with This node as the default, a count that follows the choice and a note with the backups not shown.
- Backups whose node cannot be read from their name are listed under All nodes.
- A local backup made on this node by a job that no longer exists is marked as Former job, read from its metadata file; backups without it, manual ones and those of other nodes are not marked.
- New texts in English and Spanish, modelled in the translation steady-state test.
- Tests for the node and the former job read from each backup, and for the cluster node membership notifications.

AppImage/messages/en/common.json

@@ -3953,8 +3953,15 @@
       "emptyAfter": "backups yet.",
       "emptyBefore": "No",
       "emptyMessage": "No backup archives found.",
+      "hiddenOtherNodes": "Backups of other nodes not shown: {count}.",
+      "scopeAllNodes": "All nodes",
+      "scopeAriaLabel": "Backups to show",
+      "scopeShowAll": "Show all",
+      "scopeThisNode": "This node",
       "encrypted": "Encrypted",
       "encryptedTitle": "This backup is encrypted",
+      "formerJob": "Former job",
+      "formerJobTitle": "The job that made this backup no longer exists on this node. The backup is kept until it is deleted.",
       "filesInBackup": "Files in backup",
       "group": "Group",
       "host": "Host",

AppImage/messages/es/common.json

@@ -3953,8 +3953,15 @@
       "downloadTitle": "Descargar este backup",
       "emptyAfter": "backups todavía.",
       "emptyBefore": "No",
+      "hiddenOtherNodes": "Backups de otros nodos no mostrados: {count}.",
+      "scopeAllNodes": "Todos los nodos",
+      "scopeAriaLabel": "Backups que se muestran",
+      "scopeShowAll": "Mostrar todos",
+      "scopeThisNode": "Este nodo",
       "encrypted": "cifrado",
       "encryptedTitle": "Este backup está cifrado",
+      "formerJob": "Trabajo anterior",
+      "formerJobTitle": "El trabajo que hizo este backup ya no existe en este nodo. El backup se conserva hasta que se elimine.",
       "filesInBackup": "Archivos del backup",
       "group": "Grupo",
       "host": "Host",