For now, only add the MozMenuPopup base class to MozElements,
and don't define a custom element for it with
`customElements.define`. This is to help avoid conflicts in
de-xbl work. (See the bug for details.)
Includes a function to do 'manual slotting', moving child
elements into place. Dynamically adding, modifying, or
removing child nodes after the element is connected needs
to be handled manually as well.
Differential Revision: https://phabricator.services.mozilla.com/D25467
--HG--
rename : toolkit/content/widgets/popup.xml => toolkit/content/widgets/menupopup.js
extra : moz-landing-system : lando
c3a3d09307