Make it possible to define LVGL config in a higher level format

[rafaelcaricio]

Would be nice to define library properties in a high level format than lv_conf.h file. Possibly, a lv_conf.toml would be a better approach. We could do validations and consistency checks at build time (build.rs) and generate the final lv_conf.h dynamically right before compilation.

An example content of how the lv_conf.toml could look like:

# ====================
#  Graphical settings
# ====================
[graphical]
# Maximal horizontal and vertical resolution to support by the library.
hor_res_max=480
ver_res_max=320

Users would be required to define only what differs from the default values. That way, it is easier to spot differences or custom configurations.