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

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

src/Battling/Models/BattleSide.cpp

@@ -10,7 +10,7 @@ bool BattleSide::AllPossibleSlotsFilled() const {
     try {
         for (size_t i = 0; i < _creatures.Count(); i++) {
             auto c = _creatures[i];
-            if (!c.HasValue() || c.GetValue()->IsFainted()) {
+            if (!c.HasValue() || !c.GetValue()->IsUsable()) {
                 if (_battle->CanSlotBeFilled(_index, i)) {
                     return false;
                 }