WItemSlot: Add getters for native slots and slot ids

Closes #247.

src/main/java/io/github/cottonmc/cotton/gui/widget/WItemSlot.java

@@ -26,8 +26,10 @@
 import io.github.cottonmc.cotton.gui.widget.focus.FocusModel;
 import io.github.cottonmc.cotton.gui.widget.icon.Icon;
 import org.jetbrains.annotations.Nullable;
+import org.jetbrains.annotations.UnmodifiableView;
 
 import java.util.ArrayList;
+import java.util.Collections;
 import java.util.HashSet;
 import java.util.List;
 import java.util.Objects;
@@ -409,6 +411,25 @@ protected ValidatedSlot createSlotPeer(Inventory inventory, int index, int x, in
 		return new ValidatedSlot(inventory, index, x, y);
 	}
 
+	/**
+	 * {@return an unmodifiable list containing the current slot peers}
+	 *
+	 * @since 11.1.0
+	 */
+	public @UnmodifiableView List<? extends ValidatedSlot> getPeers() {
+		return Collections.unmodifiableList(peers);
+	}
+
+	/**
+	 * Gets the starting {@linkplain net.minecraft.screen.slot.Slot#id ID} for the slot peers.
+	 *
+	 * @return the starting ID for the slot peers, or -1 if this slot widget has no peers
+	 * @since 11.1.0
+	 */
+	public int getPeerStartId() {
+		return !peers.isEmpty() ? peers.getFirst().id : -1;
+	}
+
 	/**
 	 * Gets this slot widget's background painter.
 	 *