Implement powerMonitor 'shutdown' event for Linux.

The event is emitted when the OS is rebooting/shutting down, and allows
an electron app to call `e.preventDefault()` in order to delay shutdown
and exit cleanly.

lib/browser/api/power-monitor.js

@@ -5,4 +5,16 @@ const {powerMonitor, PowerMonitor} = process.atomBinding('power_monitor')
 Object.setPrototypeOf(PowerMonitor.prototype, EventEmitter.prototype)
 EventEmitter.call(powerMonitor)
 
+powerMonitor.on('newListener', (event) => {
+  if (event === 'shutdown' && powerMonitor.listenerCount('shutdown') === 0) {
+    powerMonitor.blockShutdown()
+  }
+})
+
+powerMonitor.on('removeListener', (event) => {
+  if (event === 'shutdown' && powerMonitor.listenerCount('shutdown') === 0) {
+    powerMonitor.unblockShutdown()
+  }
+})
+
 module.exports = powerMonitor