ui: add date column in per-guest backup archive panel

Similar to commit f8211903de77e7e7f0959aeb10143f8a3129410b which
added this column to the storage content grid let's also add it here.
As we also base off the 'pve-storage-content' it's just a matter of
copying over the column definition 1:1

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>

www/manager6/grid/BackupView.js

@@ -205,6 +205,11 @@ Ext.define('PVE.grid.BackupView', {
 		    renderer: PVE.Utils.render_storage_content,
 		    dataIndex: 'volid'
 		},
+		{
+		    header: gettext('Date'),
+		    width: 150,
+		    dataIndex: 'vdate'
+		},
 		{
 		    header: gettext('Format'),
 		    width: 100,