Change Karthus' ult and Echo interaction

As mentioned in the post, ult along with unique passive Echo always target the earliest-pick player on the enemy team.

As of patch 6.23, it still works like this ( just tested ) and that's broken, as far as in-game mechanics is concerned, it is literally an arbitrary target. This is actually a bug and it should be treated as such.

It should have priority that have meaning in-game, something like closest enemy or slightly weirder like least health enemy or anything that can actually be reasoned about in-game, not some crazy random pick-order.

PS yes it will require some change in code, but this is pretty minimal, its just an ordering. Not to mention there is already ult to pull from if closest enemy makes sense.