Briton structures are located under art/actors/britons/, but for the Gauls, some are located under art/actors/celts/ (e.g. wall_gate.xml), whereas others are located under art/actors/gauls/ (e.g. all other walls). This is quite confusing.
This patch:
- moves actors used only by gaul structures from celts/ to gauls/;
- keeps actors that are used shared by brit and gaul under celts/ (e.g. outpost.xml);
- keeps actors that are unused under celts/;
- renames plot_corral.xml to corral.xml for consistency (see other civs);
- renames special.xml to rotary_mill.xml for clarity;
- corrects simulation templates accordingly.
- [EDIT] deletes britons/kennel.xml, which is an unused duplicate of celts/kennel.xml, rename the latter to celts/houselet.xml, which better describes how it looks like, and correct the only file that uses it, the migration.xml scenario.