Separate the rendering and widgets

[AlixANNERAUD]

Introduce the problem

It would be good to be able to get rid of the rendering part when needed (with a #define in lv_conf.h), somewhat like having a "client" side and a "server" side.

Proposal

The client side would only be responsible for setting up and configuring the data structure structures, and the server side for the rendering and input device. Currently, LVGL compiled in WASM occupies ~600 KB, which is significant. I think we can reduce this by getting rid of the rendering part.