Lighter window background

Native windows get lighter when they’re backgrounded so they stand out
less (at least on OS X). Let’s do this too.

Even the Spotify app, which has dozens of developers gets this wrong.
We’re so awesome :)

Also:

- Renamed a bunch of state variables (next time will make separate
commit, sry)
- All window-related variables (e.g. isFullScreen, isFocused, etc.)
live in `state.window` now
- Remove negative class name, use CSS :not() instead

renderer/views/header.js

@@ -30,7 +30,7 @@ function Header (state, dispatch) {
 
   function getTitle () {
     if (process.platform === 'darwin') {
-      return hx`<div class='title'>${state.title}</div>`
+      return hx`<div class='title'>${state.window.title}</div>`
     }
   }