Combat, do not allow to cast a spell on a troop if this spell does not affect this troop

[LeHerosInconnu]

I suggest to not allow to cast a spell on a troop if this spell does not affect this troop.

Let's take the case of the "Disrupting Ray" spell.
(I haven't thought about all the possible cases, it's just to illustrate the principle.)

We have seen here (#4845) that:

"- There is no advantage to cast the "Disrupting Ray" spell if all opposing troops are already at the minimum possible defense, so it should not be possible to cast this spell in this case."

But there may be a case where the spell can be beneficial to be cast on one troop but not on another (which is already at the minimum possible defense).
So the spell can be cast, it is not grayed out in the Magic Book, but it will not be useful for this troop which is already at the minimum defense level.
So it should not be possible to cast the spell on this troop. The spell cursor should therefore be changed to indicate to the player that the spell cannot be cast on this troop when the player positions the spell cursor on this troop.

In fheroes2: