For example, in the following HTML:
```html
<label>
<input type="radio" name="fruit" value="apple" id="radio1">
<span class="box"></span>
</label>
```
When any descendant of a <label> element is clicked, a "click" event
must be dispatched on the <input> element nested within the <label>, in
addition to the "click" event dispatched on the clicked descendant.
Previously, this behavior was implemented only for text node descendants
by "overriding" the mouse event target using `mouse_event_target()` in
the TextPaintable. This approach was incorrect because it was limited to
text nodes, whereas the behavior should apply to any box. Moreover, the
"click" event for the input control must be dispatched *in addition* to
the event on the clicked element, rather than redirecting it.
For a while we used the wider Paintable type for stacking context,
because it was allowed to be created by InlinePaintable and
PaintableBox. Now, when InlinePaintable type is gone, we can use more
specific PaintableBox type for a stacking context.
Resulting in a massive rename across almost everywhere! Alongside the
namespace change, we now have the following names:
* JS::NonnullGCPtr -> GC::Ref
* JS::GCPtr -> GC::Ptr
* JS::HeapFunction -> GC::Function
* JS::CellImpl -> GC::Cell
* JS::Handle -> GC::Root
