Add basic support of Java<->Native callbacks

Implement them for ImGuiIO::GetClipboardTextFn and ImGuiIO::SetClipboardTextFn

imgui-binding/src/main/java/imgui/callbacks/ImStrConsumer.java

@@ -0,0 +1,11 @@
+package imgui.callbacks;
+
+import java.util.function.Consumer;
+
+/**
+ * Callback to supply Java string form native code.
+ */
+public abstract class ImStrConsumer implements Consumer<String> {
+    @Override
+    public abstract void accept(String str);
+}

imgui-binding/src/main/java/imgui/callbacks/ImStrSupplier.java

@@ -0,0 +1,11 @@
+package imgui.callbacks;
+
+import java.util.function.Supplier;
+
+/**
+ * Callback to get Java string from native code.
+ */
+public abstract class ImStrSupplier implements Supplier<String> {
+    @Override
+    public abstract String get();
+}