feature: support custom attack animations via 'attack_ability' item property

[antoniaelsen]

Implements #530

[jjppof]

Hello! Thanks for taking some time to start taking a look at this!
First of all, I think my description in the bug is a bit misleading (perhaps wrong), sorry for this. So will clarify.

The idea here is that depending on the weapon a char is using, the char will perform a different animation when attacking. So basically, when defining which battle animation to pick in set_action_animation_settings method, the engine should check if there's a custom_battle_animation_key (name suggestion for the prop). If yes, then pick this as the final battle_animation_key.

I think you make this new setting to have precedence over caster animation variation (you'll see it inside set_action_animation_settings).

I hope it's clear now.

Many thanks!

[jjppof]

Oh, one more thing, there's no need to set the new prop with empty value in the json file. In the future, I intend to keep this file as clean as possible.

Thanks