Mct4170 - Give indicators priorities (#4322)

* added priority api with an enum for sane priority defaults
openmct.priority.LOW: -1000. Should always run last.
openmct.priority.DEFAULT: 0. This is the default unless you specify something else.
openmct.priority.HIGH: 1000. Should always run first.

* allow indicators to be sorted by priority

src/api/priority/PriorityAPI.js

@@ -0,0 +1,28 @@
+/*****************************************************************************
+ * Open MCT, Copyright (c) 2014-2021, United States Government
+ * as represented by the Administrator of the National Aeronautics and Space
+ * Administration. All rights reserved.
+ *
+ * Open MCT is licensed under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ * http://www.apache.org/licenses/LICENSE-2.0.
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+ * License for the specific language governing permissions and limitations
+ * under the License.
+ *
+ * Open MCT includes source code licensed under additional open source
+ * licenses. See the Open Source Licenses file (LICENSES.md) included with
+ * this source code distribution or the Licensing information page available
+ * at runtime from the About dialog for additional information.
+ *****************************************************************************/
+
+const PRIORITIES = Object.freeze({
+    HIGH: 1000,
+    DEFAULT: 0,
+    LOW: -1000
+});
+export default PRIORITIES;