Adds drag behaviour for foe actors. Dragging a compendium foe actor on to a character now behaves similarly to dragging a foe progress item did in the past.

• the drop target component now take an array of types rather than a single type
• removed foe progress item packs, because foe actors now do everything they were needed for
• the foe actor packs now use the same pack name and _ids as the foe progress items did -- which hopefully means compendium links for e.g. delve sites won't break
• strips some duplicate data (like system.rank and system.description) from foe actors, as it didn't appear to be accessed anyways; this information is now included with the child progress track
• make the code that imports foes from Dataforged DRYer
• track dataforged IDs for foes with a new key, system.dfid
• all Actor compendiums now index system.dfid

All told, this trims the packs folder from 2.9 MiB down to 2.6 MiB.