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/
* deletes `celts/kennel.xml`, which is an unused duplicate of `britons/kennel.xml`
* renames `plot_corral.xml` to `corral.xml` for consistency (see other civs)
* renames `special.xml` to `rotary_mill.xml` for clarity