This logic was kept in the GlobalEventHandlers mixing for sharing
between Document and HTMLElement, but there are other interfaces who
need to support `onfoo` attribute event listeners as well.
SPDX License Identifiers are a more compact / standardized
way of representing file license information.
See: https://spdx.dev/resources/use/#identifiers
This was done with the `ambr` search and replace tool.
ambr --no-parent-ignore --key-from-file --rep-from-file key.txt rep.txt *
The internal C++ function will now receive a RefPtr<EventListener> for
'EventListener?' and a NonnullRefPtr<EventListener> for 'EventListener'.
Examples of this are addEventListener() and removeEventListener(), which
both have nullable callback parameters.
