This refactors the object_h struct to store objects as indexes rather than pointers. This is a prerequisite for making the collection of objects dynamic in the future. See also #6100.
Follow-up to #5800 and #5829.
Several handle types in the scripting API used inheritance where composition would be more appropriate. This refactors those types, as well as removing the various handle types that were just aliases for `model_h`. These are just the C++-side types; all the Lua types remain as-is.
This also refactors `sound_h` to move the previous `isSoundValid` to `isValid` so that `ade_set_args` will work properly for sounds with the `Lua_API_returns_nil_instead_of_invalid_object` flag. The subclassing from `sound_entry_h` was removed and the "full validity check" was moved to `isValidWithEntry`. The Lua side remains unaffected, but additional documentation was added to clarify the usage.
Follow-up to #6009 and prerequisite to #6006 due to the `isValid` refactor for `sound_h`.