Ensure app is single-instance

Remove some comments.

// FREEBIE

main.js

@@ -1,9 +1,6 @@
 const electron = require('electron')
-// Module to control application life.
 const app = electron.app
-// Module to create native browser window.
 const BrowserWindow = electron.BrowserWindow
-
 const path = require('path')
 const url = require('url')
 const autoUpdater = require('electron-updater').autoUpdater
@@ -11,6 +8,20 @@ const autoUpdaterInterval = 60 * 60 * 1000;
 
 app.setAppUserModelId('org.whispersystems.signal-desktop')
 
+var shouldQuit = app.makeSingleInstance(function(commandLine, workingDirectory) {
+  // Someone tried to run a second instance, we should focus our window
+  if (mainWindow) {
+    if (mainWindow.isMinimized()) mainWindow.restore();
+    mainWindow.focus();
+  }
+  return true;
+});
+
+if (shouldQuit) {
+  app.quit();
+  return;
+}
+
 // Keep a global reference of the window object, if you don't, the window will
 // be closed automatically when the JavaScript object is garbage collected.
 let mainWindow