[GTK4] Disable Clipboard.getAvailableTypes()

This method is using GTK3-specific methods and therefore can't be used
in a GTK4 environment. Instead, an empty array is returned.

Author: ptziegler

bundles/org.eclipse.swt/Eclipse SWT Drag and Drop/gtk/org/eclipse/swt/dnd/Clipboard.java

@@ -571,6 +571,10 @@ public TransferData[] getAvailableTypes(int clipboards) {
 	checkWidget();
 
 	//TODO: [GTK4] This currently will not work in GTK4
+	if (GTK.GTK4) {
+		return new TransferData[0];
+	}
+
 	TransferData[] result = null;
 	if ((clipboards & DND.CLIPBOARD) != 0) {
 		int[] types = getAvailableClipboardTypes();