labels: allow restoring cache-map

E.g. with unexpected termination for mu it is possible that the
labels-cache (i.e., the one that is used for auto-completion) gets disconnected
from reality.

Add a --restore option to `mu label list` to restore the actual labels from the
labels seen in the store.

lib/mu-store.hh

@@ -373,6 +373,16 @@ public:
 	 */
 	Result<Labels::DeltaLabelVec> clear_labels(Message& message);
 
+	/**
+	 * Restore label-map from store
+	 *
+	 * Restore the labels list in the store, i.e., restore the cached list of labels which is
+	 * used for e.g. auto-completion in mu4e from the labels in the store.
+	 *
+	 * @return Ok or some error.
+	 */
+	Result<void> restore_label_map();
+
 	/**
 	 * Get a copy of the map of labels in use.
 	 *
@@ -382,6 +392,8 @@ public:
 	 */
 	LabelsCache::Map label_map() const;
 
+
+
 	/**
 	 * Prototype for the ForEachMessageFunc
 	 *