ui: tape/Restore: allow simple namespace mapping

add a default namespace selector (of the current default store)
and a namespace selector per target datastore (for media-sets with
multiple datastores).

to achieve that we have to change the way we handle the mapping field a bit:
* don't use it as field directly (otherwise the value gets stringified),
  but use the 'getValue' method in 'onGetValues'.
* set the defaultStore there, not only that we have one
  (with this we can now easily show it as emptytext for each store)
* add a reference to the widgets to the record so that we can access
  them in the respective change handler (also clean those references up,
  else we have a cyclic reference between record <-> widget)

in onGetValues, if we have multiple datastores, the mapping grid does
all the work for us, otherwise, we have to create the ns mapping
ourselves there.

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>

www/form/NamespaceSelector.js

@@ -66,6 +66,9 @@ Ext.define('PBS.form.NamespaceSelector', {
 	    }
 	    me.store.load();
 	    me.validate();
+	} else {
+	    me.datastore = undefined;
+	    me.setDisabled(true);
 	}
     },