Add an example lvgl v7 based Zephyr keypad widget

The following commits add a keypad widget for zephyr. So far tested in a simulator, working on deploying it on a real board.

This moves the linux main function into a separate file called main_linux.c and adds another main file: main_zephyr.c. The widget code however could be theoretically shared between zephyr and linux but we'd need to port the linux part to lvgl v7. The goal is to allow created widgets that are shared between zephyr and linux.

In the meantime: I'm also working on bringing support for lvgl v7 to meta-openembedded so that we can port the linux part.