Bug 1793359 - Simplify bookmarks popup placement. r=Itiel,dao,mak

By using delegatesanchor="true", we delegate the anchor node to the
first in-flow box (that is the icon), simplifying a lot of the
hard-coded margins in the CSS.

Do that, plus make menupopup[type="arrow"] work consistently, to
simplify the styling of the bookmarks popup a bit.

Differential Revision: https://phabricator.services.mozilla.com/D158569