Currently different civs use different actor file names to refer to the same tower structure, which is inconsistent and confusing. This patch standardizes the file names:
* `wooden_tower`, `outpost_stone`, `sentry_tower` → `tower_small`
* `fndn_wooden_tower` → `fndn_tower_small`
* `scout_tower`, `defense_tower_stone`, `defense_tower` → `tower_large`
* `fndn_defense_tower` → `fndn_tower_large`
* `babylonian_tower` → `tower_babylonian`
This is follows the same naming scheme as used by the recently added towers:
```
tower_artillery
tower_bolt
tower_double
```
and by the wall segments:
```
wall_gate
wall_long
wall_medium
wall_short
wall_tower
```
Related: D2551, D2553, D2554
Not done: remove unused `kushites/defense_tower_wood.xml` actor, which looks like the generic palisade fort.