Renamed ctt.cfg to CommunityTechTree.cfg

This is really a proposal in the form of a pull-request, so feel free to close it if you don't think it's for the best.

I've renamed `ctt.cfg` to `CommunityTechTree.cfg`. I believe this makes no difference at all to TechManager, but if means if the filename is ever viewed in isolation, it's immediately obvious what it is. :)