Bug 1041967 - [Messages] do some safe lazy load to improve launch latency

- Move scripts and css that not in the thread rendering call path into Lazyloader
- Add another callback for render thread while first panel is ready that we can lazyload/init the rest of part
- Prevent the body to receive any pointer events before all the UI modules init
nit: I think it would be good to specify the event name here. Also, I'd call "yield" after init, so that it "looks" async. So it would be this:

```javascript
  this.sinon.stub(Startup, 'on');
  Navigation.init();
  Startup.on.withArgs('post-initialize').yield();
  assert...
```
What do you think?