Member AnimationBase::add_callback (AnimationCallback callback): Need to implement removing callbacks, similar to Object class.
Class CheckBox: This should be a ValueWidget<bool>.
Member Form::m_options: This will keep pointers around to deleted child widgets.
Member Frame::add (const std::shared_ptr< Widget > &widget): Create the idea of layers by moving m_children to a 2d array. Then add a layer index (x-order) here to add widgets to different layers for drawing.
Member Gauge::add (const std::shared_ptr< GaugeLayer > &layer): This does not account for adding the same layer multiple times.
Member Image::copy (): Tricky API.
Member Image::Image (shared_cairo_surface_t surface): This is a broken API. It's different from the constructor below. Need consistency.
Member Painter::Painter (shared_cairo_t cr) noexcept: Painter needs to come from the Screen. This constructor should be hidden and you should have to get a custom version of it from the Screen for drawing. A default software/GPU painter can be created, otherwise for something like X11 we should be using X11 to paint.