This change updates the bindings generator for the case defined at
https://html.spec.whatwg.org/#reflecting-content-attributes-in-idl-attributes:element;
that is, the case “If a reflected IDL attribute has the type T?, where T
is either Element or an interface that inherits from Element”.
The change “normalizes” the generator behavior for that case — such that
the generated code expects a getter with a name of the form used in
other cases; e.g., popover_target_element().
Otherwise, without this change, the generator expects a name of the form
get_popover_target_element() for that case.
The popoverTargetElement seems to be one of the only cases of a
reflected Element? attribute in the HTML spec, the behaviour of which
is specified in section 2.6.1.
Buttons can't actually toggle popovers yet because showing/hiding
popovers is not implemented yet.