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: same here: `Startup.on.withArgs('post-initialize`).yield();`