Iterate only once over QJsonObject, create key list by
existing loop instead of create by QJsonObject::keys(),
which contains internal loop.
In common case if loop's statement is lightweight,
then effect of optimization is significant, and vice versa.
Also make addJsonArray() and addJsonObject() functions
more homogeneous.
Use reserve to optimize memory allocation.
Change-Id: Id122cd1becfd34bb06640876b1c79e1d396d2a6b
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>
69ab280315