add ability and item fix

[AaronSong321]

add file AbilityAbstraction.lua; decrease desire of target ability usage at targets with spell block; prevent target ability usage at friend illusions; decrease desire of passive toggle abilities to save mana; fix terrorblade, dark_seer, antimage, necrolyte, kunkka et al. cannot use abilities; add medusa split arrow; add medusa toggle shield off to save mana; add item purchase (witch_blade, falcon_blade, holy_locket, gleipnir, wind_waker, ghost_sceptre); remove courier purchase;