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/plugins/clock/plugin.js

@@ -116,7 +116,8 @@ export default function ClockPlugin(options) {
             });
             const indicator = {
                 element: clockIndicator.$mount().$el,
-                key: 'clock-indicator'
+                key: 'clock-indicator',
+                priority: openmct.priority.LOW
             };
 
             openmct.indicators.add(indicator);