add WguiFontSystem, remove FONT_SYSTEM singleton, custom fonts, add Light font weight

there are a few gzip-compressed ttf as for now, looks like variable fonts aren't parsed properly by cosmic_text. Not sure why. Also, we probably need to have a fallback for CJK characters in the future, or just fallback to the built-in ones in the OS.

dash-frontend/src/tab/mod.rs

@@ -2,7 +2,6 @@ use std::rc::Rc;
 
 use wgui::{
 	components::button::ComponentButton,
-	event::CallbackDataCommon,
 	globals::WguiGlobals,
 	layout::{Layout, WidgetID},
 };