Show spell damage to target when hovering over them with spell selected

After selecting a damaging spell, show exactly how much damage it'd deal to the targeted stack similarly to the way potential damage from troop attacks are displayed.

Artist's high-quality depiction: https://i.imgur.com/XwvWq9Q.png

This could also include a more detailed breakdown of how the damage is calculated, such as base spell damage, Spell Damage Power bonus damage, mitigation from Magic Resist, etc.

As for AoE damage spells, adding the same mouse-over method would be sufficient as displaying the damage tooltip on affected troops is not always possible (think Chain Lightning).