Added screenshots for some examples
* showcase: an overview of all widgets provided by OrbTk
* calculator: calculator example
+ ![calculator](https://gitlab.redox-os.org/kivimango/assets/-/raw/orbtk_examples/screenshots/Calculator.png)
* canvas: use third party render library in canvas
+ ![canvas](https://gitlab.redox-os.org/kivimango/assets/-/raw/orbtk_examples/screenshots/orbtk_examples/canvas_example.jpg)
* login: PasswordBox showcase with a login form
* minimal: minimal example
* multi_window: multi window example
+ ![multi_window](https://gitlab.redox-os.org/kivimango/assets/-/raw/orbtk_examples/screenshots/orbtk_examples/multi_window.jpg)
* overlay: draw widgets on the top
+ ![overlay](https://gitlab.redox-os.org/kivimango/assets/-/raw/orbtk_examples/screenshots/orbtk_examples/overlay_example.jpg)
* popup: show how to open and use a popup
* settings: use registry and settings service (load / save)
+ ![settings](https://gitlab.redox-os.org/kivimango/assets/-/raw/orbtk_examples/screenshots/orbtk_examples/settings_example.jpg)
* stack: stack layout example
* widgets: widget overview
