Fix plKeyCollector key trashing...

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.

Removing the rest of the hsTArray<plKey> is pending. That's a lot of spidery work that (unfortunately) goes deep inside the Max plugin. Let's go ahead and get this in.