Add an id to activities without id when storing them.

src/timeline.js

@@ -5,12 +5,22 @@ import {Activity} from './activity.js'
 const KnownActivities = {
   // Cache
   activities: {},
+  // Counter if id is unset
+  count: 0,
   // Methods
   get: function(id) {
     return KnownActivities.activities[id]
   },
   set: function(id, obj) {
-    KnownActivities.activities[id] = obj
+    // An id is needed
+    if (id) {
+      KnownActivities.activities[id] = obj
+    } else {
+      // Use the counter to compute an id
+      KnownActivities.count = KnownActivities.count + 1
+      obj.id = '' + KnownActivities.count
+      KnownActivities.activities[obj.id] = obj
+    }
   }
 }