Fix warning in ListIndexModel and add testcase

modules/gio/src/main/java/io/github/jwharm/javagi/gio/ListIndexModel.java

@@ -80,12 +80,16 @@ public void setSize(int size) {
      * Returns the gtype of {@link ListIndex}
      * @return always returns the value of {@link ListIndex#gtype}
      */
-    @Property(name="item-type", writable=false)
+    @Property(name="item-type", constructOnly = true)
     @Override
     public Type getItemType() {
         return ListIndex.gtype;
     }
 
+    @Property(name="item-type")
+    public void setItemType(Type itemType) {
+    }
+
     /**
      * Returns the size of the list model
      * @return the value of the size field

modules/gio/src/test/java/io/github/jwharm/javagi/test/gio/ListModelTest.java

@@ -0,0 +1,28 @@
+package io.github.jwharm.javagi.test.gio;
+
+import io.github.jwharm.javagi.gio.ListIndexModel;
+import org.gnome.gio.Gio;
+import org.gnome.gio.ListModel;
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+/**
+ * Test some operations on ListIndexModel
+ */
+public class ListModelTest {
+
+    @Test
+    public void createListModel() {
+        // explicitly trigger initialization
+        Gio.javagi$ensureInitialized();
+
+        ListModel listIndexModel = ListIndexModel.newInstance(1000);
+        assertEquals(listIndexModel.getItemType(), ListIndexModel.ListIndex.getType());
+        assertEquals(1000, listIndexModel.getNItems());
+
+        var item500 = (ListIndexModel.ListIndex) listIndexModel.getItem(500);
+        assertNotNull(item500);
+        assertEquals(500, item500.getIndex());
+    }
+}