[battle_engine] Several issues with mega evolution

[kleeenexfeu]

All of what's mentionned below is fixed in this PR.

According to https://bulbapedia.bulbagarden.net/wiki/Mega_Evolution switching should happen before mega evolving.

As of now it doesn't change much, because the mid-turn speed recalculation aren't implemented (except for #1914 which might include said changes in the future), but when they are, this inconsistency will matter.

[kleeenexfeu]

Also, the mega evolutions occurs in the wrong order, because of this :
https://github.com/rh-hideout/pokeemerald-expansion/blob/battle_engine/src/battle_main.c#L4676-L4677
The mega evolution is checked for battlerId 0 first (first player's mon), then battlerId 1 (first AI's mon), etc, without taking into account speed/priority.
It can have a great impact on the battle when mega evolving makes you loose/gain a switch in ability, for instance.

[kleeenexfeu]

Mega Evolution wish causes other mega evolution to display the fervent wish message because the isWishMegaEvo field of MegaEvolutionData is only a bool. And once it's set I didn't see a place where it was cleared, letting the game think that all the mega evolutions that follow are wish as well. This field should be 1 bool per battler, even one byte, or more carefully cleared/set when it should. I'll try to fix these issues in a single PR.