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/api.js

@@ -31,7 +31,8 @@ define([
     './Editor',
     './menu/MenuAPI',
     './actions/ActionsAPI',
-    './status/StatusAPI'
+    './status/StatusAPI',
+    './priority/PriorityAPI'
 ], function (
     TimeAPI,
     ObjectAPI,
@@ -43,7 +44,8 @@ define([
     EditorAPI,
     MenuAPI,
     ActionsAPI,
-    StatusAPI
+    StatusAPI,
+    PriorityAPI
 ) {
     return {
         TimeAPI: TimeAPI.default,
@@ -56,6 +58,7 @@ define([
         EditorAPI: EditorAPI,
         MenuAPI: MenuAPI.default,
         ActionsAPI: ActionsAPI.default,
-        StatusAPI: StatusAPI.default
+        StatusAPI: StatusAPI.default,
+        PriorityAPI: PriorityAPI.default
     };
 });