This makes the specializations responsible to register the environment aspect
that makes the most sense for them.

The only real user of this is the QmlProject, which added its own
EnvironmentAspect on top of LocalEnvironmentAspect set by the base
class.

Change-Id: I2ad8c23a008e249dc919491a5fd397ec04502375
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>