add LittlevGL demo

add LittlevGL demo on stm32f746 discovery board

components/gui/LittlevGL/3rdparty/lv_examples/README.md

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+# Examples for Littlev Graphics Library
+
+LittlevGL is a free and open-source graphics library providing everything you need to create a Graphical User Interface (GUI) on embedded systems with easy-to-use graphical elements, beautiful visual effects and low memory footprint.
+
+GitHub: https://github.com/littlevgl/lvgl
+Website: https://littlevgl.com
+
+## Add the examples to your projects
+1. Clone this repository: `git clone https://github.com/littlevgl/lv_examples.git` or download from the [Download page](https://littlevgl.com/download). To always use the newest version the cloning is recommended.
+2. The `lv_examples` directory should be next to the `lvgl` directory in your project.
+
+Similary to `lv_conf.h` there is a configuration file for the examples too. It is called `lv_ex_conf.h`.
+1. Copy `lv_examples/lv_ex-conf_templ.h` next to `lv_examples` directory
+2. Rename is to `lv_ex_conf.h`
+3. Delete the first `#if` and  last `#endif` to enable the file's content
+4. Enable or Disable modules
+
+## Tutorial
+A step-by-step guide to teach the most important parts of the Graphics Library.
+* Hello world
+* Objects (graphical components)
+* Styles
+* Themes
+* Anti-aliasing
+* Images
+* Fonts
+* Animations
+* Responsive
+* Keyboard
+
+## Applications
+Real life GUI examples which can be adapted in your own project. The applications are designed to adapt to your screen's resolution.
+* Demo
+* Benchmark
+* System performance monitor
+* Touchpad calibrator
+* Terminal
+
+## Tests
+Test cases to validate the features of LittelvGL. You can also use them as examples. The most important and useful tests are:
+* Theme test: `lv_test_theme_1()`
+* Keyboard interface test: `lv_test_group_1()`
+* Tests of object types: `lv_test_..._1/2/3()` e.g. (lv_test_slider_1())
+
+## Contributing
+For contribution and coding style guidelines, please refer to the file docs/CONTRIBUTNG.md in the main LittlevGL repo:
+  https://github.com/littlevgl/lvgl
+You are encouraged to use the 'astyle' util to format your code prior to pushing it. The files docs/astyle_(c/h) contain astyle rules to format source and header files according to the project coding guidelines.
+