Add basic support of Java<->Native callbacks

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

imgui-binding/src/main/native/jni_callbacks.cpp

@@ -0,0 +1,23 @@
+#include "jni_callbacks.h"
+
+jmethodID jImStrConsumerAcceptMID;
+jmethodID jImStrSupplierGetMID;
+
+namespace Jni
+{
+void InitCallbacks(JNIEnv* env) {
+    jclass jImStrConsumer = env->FindClass("imgui/callbacks/ImStrConsumer");
+    jImStrConsumerAcceptMID = env->GetMethodID(jImStrConsumer, "accept", "(Ljava/lang/String;)V");
+
+    jclass jImStrSupplier = env->FindClass("imgui/callbacks/ImStrSupplier");
+    jImStrSupplierGetMID = env->GetMethodID(jImStrSupplier, "get", "()Ljava/lang/String;");
+}
+
+void CallImStrConsumer(JNIEnv* env, jobject consumer, const char* str) {
+    env->CallVoidMethod(consumer, jImStrConsumerAcceptMID, env->NewStringUTF(str));
+}
+
+jstring CallImStrSupplier(JNIEnv* env, jobject supplier) {
+    return (jstring)env->CallObjectMethod(supplier, jImStrSupplierGetMID);
+}
+}