Text <input> fields will now generate a basic shadow DOM and attach it
to the input element.
The shadow DOM contains a <div> with some inline style, and an always-
editable text node inside it. Accessing the "value" attribute on such
an input element will get/set the value from that text node.
This is really cool, although not super stable since HTML editing is
not super stable. But it's a start! :^)
The approach of attaching sub-widgets to the web view widget was only
ever going to work in single-process mode, and that's not what we're
about anymore, so let's just get rid of WidgetBox so we don't have the
dead-end architecture hanging over us.
The next step here is to re-implement <input type=text> using LibWeb
primitives.
This is a workaround until we can implement a proper <input type=text>
in terms of LibWeb primitives.
This makes google.com not crash in multi-process mode (but there is no
search box.)
