The clean-up in 71c457c36e turns out to
not be enough for all cases. So be more thorough and clear up anything
that could affect the next test.
In particular this fixes flakiness in `Text/input/select-text.html` but
hopefully it solves the issue for good!
This allows dragging elements on the page and dropping them onto other
elements. This does not yet support dragging text.
The test added here is manual; the WPT tests rely heavily on WebDriver
actions.
The spec dictates that dragenter events must be cancelled in order for
drops to be accepted on the entered element. Web reality disagrees, as
all three major browsers do not have this requirement.
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
