QDoc: minor cleanup in Generator and Node

Generator:
* Don't sort container of QMap's keys as they're already sorted. The
  container can thus be made const.

Node:
* Make getter const
* Remove qAsConst used in a ranged for loop in a const method.

Both:
* Remove qAsConst rendered superfluous.

Change-Id: I8f65f87c0ed372573cdd17f96e95cd2732effdd7
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>