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
* footprint, rubble, obstruction, and visual actors are not included in the generic template, because these belong in civ specific templates
* changes generic name and class to Stable (singular, without s)
See also D1031.