When setting the textContent of an element with no children to null or
the empty string, nothing happens. Even so, we were still invalidating
style, layout and collections, causing pointless churn.
Skipping invalidation in this case also revealed that we were missing
invalidation when changing the selected state of HTMLOptionElement.
This was all caught by existing tests already in-tree. :^)
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
This shouldn't just be a simple reflection of the label attribute.
It also needs fallback to the HTMLOptionElement.text property if the
label attribute is absent.
We currently have 2 virtual methods to inform DOM::Element subclasses
when an attribute has changed, one of which is spec-compliant. This
patch removes the non-compliant variant.
