[LVGL] Add support for bottom, top, and system layers

[AndyEveritt]

Please can support for creating bottom, top, and system layers in the designer be added.
https://docs.lvgl.io/master/details/main-components/display.html#screen-layers

[mvladic]

I'm closing #638, since this feature request is more general.

Here is my comment from the #638:

"We can do this by marking some page as top layer and every widget in that page will be added to the top_layer. Studio will make the rest so that this works in both simulator and generated code."