Function fltk::app::set_frame_type_cb

pub fn set_frame_type_cb(
    old_frame: FrameType,
    cb: fn(x: i32, y: i32, w: i32, h: i32, c: Color),
    x: i32,
    y: i32,
    w: i32,
    h: i32
)

Set the app’s default frame type