Windows/Linux: Fix broken Quit option in Tray icon

This also cleans up the code by handling quit in the same way for all
platforms, removing the special case in tray.js for darwin.

We already have a 'before-quit' handler in main/index.js, so this is
now handled there :)

main/index.js

@@ -1,6 +1,7 @@
 var electron = require('electron')
 
 var app = electron.app
+var ipcMain = electron.ipcMain
 
 var autoUpdater = require('./auto-updater')
 var config = require('../config')
@@ -62,8 +63,14 @@ function init () {
     processArgv(argv)
   })
 
-  app.on('before-quit', function () {
+  app.on('before-quit', function (e) {
+    if (app.isQuitting) return
+
     app.isQuitting = true
+    e.preventDefault()
+    windows.main.send('dispatch', 'saveState') /* try to save state on exit */
+    ipcMain.once('savedState', () => app.quit())
+    setTimeout(() => app.quit(), 2000) /* quit after 2 secs, at most */
   })
 
   app.on('activate', function () {