Implement 2D graphic API (#87)

* Implement 2D graphic library based on LittlevGL

* Add lvgl license file

core/iwasm/lib/native-interface/gui_api.h

@@ -0,0 +1,37 @@
+/*
+ * Copyright (C) 2019 Intel Corporation.  All rights reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#ifndef GUI_API_H_
+#define GUI_API_H_
+#include "bh_platform.h"
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+void wasm_obj_native_call(int32 func_id, uint32 argv_offset, uint32 argc);
+void wasm_btn_native_call(int32 func_id, uint32 argv_offset, uint32 argc);
+void wasm_label_native_call(int32 func_id, uint32 argv_offset, uint32 argc);
+void wasm_cb_native_call(int32 func_id, uint32 argv_offset, uint32 argc);
+void wasm_list_native_call(int32 func_id, uint32 argv_offset, uint32 argc);
+
+
+#ifdef __cplusplus
+}
+#endif
+
+
+#endif /* GUI_API_H_ */