Added configuration key to disable layout HUD.

Configuration key is "show-layout-HUD-on-space-change", added a new
entry in the default config with the value true.
Modified the initWithScreen:delegate to wrap the observe call.

I find myself switching spaces very often, and being able to turn off the HUD makes the switch quieter.
The configuration key is a bit long, but descriptive. Tell me if you prefer another one.