[GTK4] Implement correct moveAbove()/moveBelow() control behavior

Regardless of which control are given as input parameters, the controls
are currently always added as last child to their parents. Underlying
issue seems to be that the swt_fixed_restack() method does not handle
the GTK4 widgets correctly. As a solution, perform the movement using
the proper API.

To understand the combination in which GTK methods need to be called,
consider the expected behavior based on the GTK and SWT documentation:

[GTK4 Documentation]

- gtk_widget_insert_after(widget, parent, previous_sibling)

  It will be placed after previous_sibling, or at the beginning if
  previous_sibling is NULL.

- gtk_widget_insert_before(widget, parent, next_sibling)

  It will be placed before next_sibling, or at the end if next_sibling
  is NULL.

[SWT Documentation]

- moveAbove(control)

  Moves the receiver above the specified control in the drawing order.
  If the argument is null, then the receiver is moved to the top of the
  drawing order

This means that if the specified control is NULL,
gtk_widget_insert_after(...) needs to be called, otherwise
gtk_widget_insert_before(...).

- moveBelow(control)

  Moves the receiver below the specified control in the drawing order.
  If the argument is null, then the receiver is moved to the bottom of
  the drawing order.

Here the inverse applies. If the specified control is NULL,
gtk_widget_insert_before(...) needs to be called, otherwise
gtk_widget_insert_after(...).

bundles/org.eclipse.swt/Eclipse SWT PI/gtk/library/gtk4.c

@@ -2083,6 +2083,26 @@ JNIEXPORT jlong JNICALL GTK4_NATIVE(gtk_1widget_1get_1root)
 }
 #endif
 
+#ifndef NO_gtk_1widget_1insert_1after
+JNIEXPORT void JNICALL GTK4_NATIVE(gtk_1widget_1insert_1after)
+	(JNIEnv *env, jclass that, jlong arg0, jlong arg1, jlong arg2)
+{
+	GTK4_NATIVE_ENTER(env, that, gtk_1widget_1insert_1after_FUNC);
+	gtk_widget_insert_after((GtkWidget *)arg0, (GtkWidget *)arg1, (GtkWidget *)arg2);
+	GTK4_NATIVE_EXIT(env, that, gtk_1widget_1insert_1after_FUNC);
+}
+#endif
+
+#ifndef NO_gtk_1widget_1insert_1before
+JNIEXPORT void JNICALL GTK4_NATIVE(gtk_1widget_1insert_1before)
+	(JNIEnv *env, jclass that, jlong arg0, jlong arg1, jlong arg2)
+{
+	GTK4_NATIVE_ENTER(env, that, gtk_1widget_1insert_1before_FUNC);
+	gtk_widget_insert_before((GtkWidget *)arg0, (GtkWidget *)arg1, (GtkWidget *)arg2);
+	GTK4_NATIVE_EXIT(env, that, gtk_1widget_1insert_1before_FUNC);
+}
+#endif
+
 #ifndef NO_gtk_1widget_1measure
 JNIEXPORT void JNICALL GTK4_NATIVE(gtk_1widget_1measure)
 	(JNIEnv *env, jclass that, jlong arg0, jint arg1, jint arg2, jintArray arg3, jintArray arg4, jintArray arg5, jintArray arg6)

bundles/org.eclipse.swt/Eclipse SWT PI/gtk/library/gtk4_stats.h

@@ -173,6 +173,8 @@ typedef enum {
 	gtk_1widget_1get_1prev_1sibling_FUNC,
 	gtk_1widget_1get_1receives_1default_FUNC,
 	gtk_1widget_1get_1root_FUNC,
+	gtk_1widget_1insert_1after_FUNC,
+	gtk_1widget_1insert_1before_FUNC,
 	gtk_1widget_1measure_FUNC,
 	gtk_1widget_1set_1cursor_FUNC,
 	gtk_1widget_1set_1focusable_FUNC,

bundles/org.eclipse.swt/Eclipse SWT PI/gtk/org/eclipse/swt/internal/gtk4/GTK4.java

@@ -649,6 +649,18 @@ public class GTK4 {
 	 * @param allocation cast=(GtkAllocation *),flags=no_out
 	 */
 	public static final native void gtk_widget_size_allocate(long widget, GtkAllocation allocation, int baseline);
+	/**
+	 * @param widget cast=(GtkWidget *)
+	 * @param parent cast=(GtkWidget *)
+	 * @param previous_sibling cast=(GtkWidget *)
+	 */
+	public static final native void gtk_widget_insert_after(long widget, long parent, long previous_sibling);
+	/**
+	 * @param widget cast=(GtkWidget *)
+	 * @param parent cast=(GtkWidget *)
+	 * @param next_sibling cast=(GtkWidget *)
+	 */
+	public static final native void gtk_widget_insert_before(long widget, long parent, long next_sibling);
 
 	/* GtkComboBox */
 	/** @param combo_box cast=(GtkComboBox *) */

bundles/org.eclipse.swt/Eclipse SWT/gtk/org/eclipse/swt/widgets/Composite.java

@@ -1353,7 +1353,15 @@ void markLayout (boolean changed, boolean all) {
 void moveAbove (long child, long sibling) {
 	if (child == sibling) return;
 	long parentHandle = parentingHandle ();
-	OS.swt_fixed_restack (parentHandle, child, sibling, true);
+	if (GTK.GTK4) {
+		if (sibling == 0) {
+			GTK4.gtk_widget_insert_after(child, parentHandle, 0L);
+		} else {
+			GTK4.gtk_widget_insert_before(child, parentHandle, sibling);
+		}
+	} else {
+		OS.swt_fixed_restack (parentHandle, child, sibling, true);
+	}
 	return;
 }
 
@@ -1364,7 +1372,15 @@ void moveBelow (long child, long sibling) {
 		moveAbove (child, scrolledHandle != 0  ? scrolledHandle : handle);
 		return;
 	}
-	OS.swt_fixed_restack (parentHandle, child, sibling, false);
+	if (GTK.GTK4) {
+		if (sibling == 0) {
+			GTK4.gtk_widget_insert_before(child, parentHandle, 0L);
+		} else {
+			GTK4.gtk_widget_insert_after(child, parentHandle, sibling);
+		}
+	} else {
+		OS.swt_fixed_restack (parentHandle, child, sibling, false);
+	}
 	return;
 }