Add support for switching screens

[Fabi019]

This PR adds support for changing the current screen via the load_scr function based on the lv_disp_load_scr from lvgl.

Example usage:

// Default screen
let mut def_screen = ui.scr_act()?;
// Extra screen
let mut screen = Obj::default();
// ... Add style and widgets to each screen

// Load the extra screen
ui.load_scr(&mut screen);

// Change back to def_screen
ui.load_scr(&mut def_screen);

[rafaelcaricio]

Thank you! 😃 Merged, but please be aware that I am not actively maintaining this project anymore.