KConfigGroup: use QString as official group name data type

Lots of querying methods return the group name as QString type,
e.g. QStringList KCnfig*::groupList() or KConfigGroup::name().
And KConfigSkeleton* classes are using only the QString type for
group names.

Though the memory storage of KConfig is using QByteArray to store the
group name. Which results for many API calls in lots of runtime encoding
conversions including memalloc.

Standardizing on QString as official group name data type and using it
also in the memory storage model avoids those runtime conversions.