KConfigGroup::keyList(): use dedicated implementation, avoiding unused map

Reusing KConfigGroup::entryMap() came with the price paid for generating
a complete map including new deep QString instances for all the entries.
Just to discard all the entries and the map structure and create a new
list structure, only reusing the key strings.

The extended benchmark showed for a group with a single entry a reduction
of ~ 35 %, which improves more with more entries (e.g. 3 -> ~40 %).