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 about lazyloading styles, I've heard they produce reflows when you add them.

I'd rather try to concatenate them instead (manually first, to see if there is a difference, and then in a build process if it proves useful)