Add fallback font "stack" with lv_ll

[samueldr]

This would be used to add e.g. all of Noto fonts as a fallback, instead of rudely showing stupid missing char diamonds.

The API would basically be that we'd have a static linked_list ref, and prepend/append fallback fonts at runtime.

When looking for a fallback glyph, we "just" loop on the linked list until we find one.