[Timeline] Add default values to model

Add default values to model, such that editing which occurs
before user supplies these properties does not cause errors
to occur. Directly addresses #717, indirectly addresses
remaining errant behavior associated with #790.

platform/features/timeline/bundle.js

@@ -172,7 +172,10 @@ define([
                         }
                     ],
                     "model": {
-                        "composition": []
+                        "composition": [],
+                        "start": {
+                            "timestamp": 0
+                        }
                     }
                 },
                 {
@@ -211,6 +214,12 @@ define([
                         "composition": [],
                         "relationships": {
                             "modes": []
+                        },
+                        "start": {
+                            "timestamp": 0
+                        },
+                        "duration": {
+                            "timestamp": 0
                         }
                     }
                 },