chore: replace return value of inventory with inventoryGui and add so…

…me Contracts to enhance simple usage (#281)

src/main/java/minevalley/core/api/gui/MultiPageGui.java

@@ -25,6 +25,7 @@ public interface MultiPageGui extends InventoryGui {
      * @throws IllegalArgumentException if the slot is out of bounds
      */
     @Nullable
+    @Contract(pure = true)
     ItemStack getItem(@Nonnegative int page, @Nonnegative int slot) throws IllegalArgumentException;
 
     /**
@@ -35,6 +36,7 @@ public interface MultiPageGui extends InventoryGui {
      * @throws IllegalArgumentException if the slot is out of bounds
      */
     @Override
+    @Contract(pure = true)
     default @Nullable ItemStack getItem(@Nonnegative int slot) throws IllegalArgumentException {
         return getItem(0, slot);
     }
@@ -103,7 +105,8 @@ MultiPageGui setPageDependantItem(@Nonnegative int slot, @Nonnull Function<Integ
      * @throws IllegalArgumentException if the page is out of bounds
      */
     @Nonnull
-    Inventory getInventory(@Nonnegative int page) throws IllegalArgumentException;
+    @Contract("_ -> new")
+    InventoryGui getInventory(@Nonnegative int page) throws IllegalArgumentException;
 
     /**
      * Opens this GUI for the specified player on the specified page.
@@ -135,8 +138,9 @@ MultiPageGui setPageDependantItem(@Nonnegative int slot, @Nonnull Function<Integ
      * @return a copy of the underlying inventory of this GUI
      */
     @Override
+    @Contract("-> new")
     default @Nonnull Inventory getInventory() {
-        return getInventory(0);
+        return getInventory(0).getInventory();
     }
 
     /**