Fix a bug related to trashed keys.

It appears that the hsTArray memory management really sucks for smart
pointers like plKey. The crash mentioned at
http://forum.guildofwriters.org/viewtopic.php?f=117&t=6291 went away
immediately after switching plKeyCollector to an std::set.
Merge pull request #371 from Hoikas/keycol-set

Fix plKeyCollector key trashing...