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
I'm not sure for the name "ready", it's quite overloaded (see "App.setReady()" below for example). How about using `'post-initialize'` ?