Allow placing BoundItem in any gui after binding

Author: NichtStudioCode

invui/src/main/java/xyz/xenondevs/invui/gui/AbstractGui.java

@@ -550,7 +550,7 @@ public void setSlotElement(int index, @Nullable SlotElement slotElement) {
 
         // set the gui if it is a bound item
         if (slotElement instanceof SlotElement.Item(Item item)) {
-            if (item instanceof BoundItem boundItem) {
+            if (item instanceof BoundItem boundItem && !boundItem.isBound()) {
                 boundItem.bind(this);
             }
         }

invui/src/main/java/xyz/xenondevs/invui/item/AbstractBoundItem.java

@@ -21,9 +21,14 @@ public Gui getGui() {
     @Override
     public void bind(Gui gui) {
         if (this.gui != null)
-            return;
+            throw new IllegalStateException("Item is already bound to a gui");
 
         this.gui = gui;
     }
 
+    @Override
+    public boolean isBound() {
+        return gui != null;
+    }
+    
 }

invui/src/main/java/xyz/xenondevs/invui/item/BoundItem.java

@@ -28,9 +28,17 @@ public sealed interface BoundItem extends Item permits AbstractBoundItem {
      * Called when the item is added to a {@link Gui}.
      *
      * @param gui The {@link Gui} to bind this item to.
+     * @throws IllegalStateException If this item is already bound to a {@link Gui}.
      */
     void bind(Gui gui);
 
+    /**
+     * Checks if this item is already bound to a {@link Gui}.
+     *
+     * @return Whether this item is already bound to a {@link Gui}.
+     */
+    boolean isBound();
+    
     static Builder<Gui> gui() {
         return new CustomBoundItem.Builder<>();
     }
@@ -48,7 +56,7 @@ static Builder<TabGui> tabGui() {
     }
 
     sealed interface Builder<G extends Gui> extends Item.Builder<Builder<G>> permits CustomBoundItem.Builder {
-
+        
         Builder<G> addBindHandler(BiConsumer<Item, G> handler);
 
         Builder<G> addClickHandler(TriConsumer<Item, G, Click> handler);