The recent introduction of the artillery and bolt towers messed up some things, because the new templates were carelessly copied from the scout/stone/defense tower templates. This patch tries to bring some order to the `template_structure_defensive_tower*` templates and does the following:
* file moves
* renames `*_sentry.xml` to `*_small.xml` and updates all child templates accordingly
* renames `*_stone.xml` to `*_large.xml` and updates all child templates accordingly
* renames the two old icons for consistency with the new ones (`tower_artillery.png` and `tower_bolt.png`)
* classes:
* makes the Defensive and Tower classes visible (cf. Civic, Economic, Military, Resource)
* deletes the unused GarrisonTower class
* replaces the now problematic DefenseTower visible class
* ensures each generic template has exactly one corresponding visible class (ArtilleryTower, BoltTower, LargeTower, SmallTower, Outpost)
* updates the AI files accordingly
* updates the technology files to ensure arrow technologies don't apply to the artillery and bolt towers
* simplifies the build restrictions