Add basic support of Java<->Native callbacks

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

imgui-binding/src/main/native/jni_common.h

@@ -8,6 +8,8 @@ namespace Jni
 {
 void InitCommon(JNIEnv* env);
 
+JNIEnv* GetEnv();
+
 void ImVec2Cpy(JNIEnv* env, ImVec2* src, jobject dst);
 void ImVec2Cpy(JNIEnv* env, ImVec2 src, jobject dst);