[Framework] Add extension sorter

Add priority ordering to loaded extensions in each category;
this allows control over the resulting order of extensions
acquired and used within the application. WTD-590

platform/framework/src/Constants.js

@@ -16,5 +16,13 @@ define({
         "tests": "test",
         "configuration": {},
         "extensions": {}
-    }
+    },
+    PRIORITY_LEVELS: {
+        "fallback": Number.NEGATIVE_INFINITY,
+        "default": 100,
+        "optional": 200,
+        "preferred": 400,
+        "mandatory": Number.POSITIVE_INFINITY
+    },
+    DEFAULT_PRIORITY: 0
 });