Add new Creature::IsUsable function, which replaces IsFainted functionality in several places.

Signed-off-by: Deukhoofd <Deukhoofd@gmail.com>

src/Battling/Flow/TurnHandler.cpp

@@ -50,8 +50,8 @@ void TurnHandler::ExecuteChoice(const ArbUt::BorrowedPtr<BaseTurnChoice>& choice
     if (battle->HasEnded()) {
         return;
     }
-    // If the user is fainted, we don't want to execute its choice.
-    if (user->IsFainted()) {
+    // If the user is not usable, we don't want to execute its choice.
+    if (!user->IsUsable()) {
         return;
     }
     // If the user is not in the field, we don't want to execute its choice.