Currently the (cavalry) stable template is a child of the barracks template, which is odd; the elephant stable, dog kennel, and workshop all inherit from the military template, not from the barracks template.
This patch:
* moves `template_structure_military_barracks_stables.xml` to `template_structure_military_stable.xml`
* updates all its children accordingly
* merges values from `pers_stable` into the new generic template
* changes generic name Stable (singular, without s)
* footprint and obstruction are removed in the civ specific templates, unless they're actually different.
* standardizes <Footprint/Height> to 12.0 (kush and pers had different values)
* standardizes foundation actor and rubble sizes to 5x5 (pers had 4x4)