Add font loader for Sandbox, Program and Application.

[genusistimelord]

This will add a Sync based font loader that will load all custom fonts placed inside the settings under custom_fonts.

#Example

fn main() -> iced::Result {
    let mut settings = Settings::default();
    settings = settings.add_custom_fonts([iced_aw::graphics::icons::ICON_FONT_BYTES]);
    CardExample::run(settings)
}

[hecrj]

API changes need an RFC or, at the very least, some coordination!

I have other plans for this.