startup perf: Reduce require() calls

Every require() that we do before the users sees UI reduces startup
time.

I used the following code (added to index.js) to log every require()
call in the main process:

```js
var Module = require('module')
var required = {}
Module.prototype.require = function (orig) {
  return function (id) {
    if (!required[id]) {
      required[id] = true
      console.log(`${id}   (from ${this.filename})`)
    }
    return orig.apply(this, arguments)
  }
}(Module.prototype.require)
```

From this I was able to learn that lots of modules were being required
that aren't actually used until later.

I also sent this related PR to eliminate another few require()s:
https://github.com/LinusU/node-application-config/pull/4

This increases startup time by 50ms.

We'll probably realize much bigger gains by following this same
procedure for the renderer process.

src/main/windows/main.js

@@ -21,7 +21,6 @@ const app = electron.app
 const config = require('../../config')
 const log = require('../log')
 const menu = require('../menu')
-const tray = require('../tray')
 
 function init (state, options) {
   if (main.win) {
@@ -81,6 +80,8 @@ function init (state, options) {
   })
 
   win.on('close', function (e) {
+    const tray = require('../tray')
+
     if (process.platform !== 'darwin' && !tray.hasTray()) {
       app.quit()
     } else if (!app.isQuitting) {
@@ -221,11 +222,15 @@ function toggleFullScreen (flag) {
 }
 
 function onWindowBlur () {
+  const tray = require('../tray')
+
   menu.setWindowFocus(false)
   tray.setWindowFocus(false)
 }
 
 function onWindowFocus () {
+  const tray = require('../tray')
+
   menu.setWindowFocus(true)
   tray.setWindowFocus(true)
 }